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Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U(NR)(2)X(4)](2-) (X = Cl(-), Br(-)) and non-coordinating NEt(4)(+) and PPh(4)(+) countercations are reported. In general, these compounds can be prepared from U(NR)(2)I(2)(THF)(x) (x = 2 and R = (t)Bu, Ph; x = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl(-), the [U(NMe)(2)](2+) cation also reacts with Br(-) to form stable [NEt(4)](2)[U(NMe)(2)Br(4)] complexes. These materials were used as a platform to compare electronic structure and bonding in [U(NR)(2)](2+) with [UO(2)](2+). Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U-Cl bonding interactions in [PPh(4)](2)[U(N(t)Bu)(2)Cl(4)] and [PPh(4)](2)[UO(2)Cl(4)]. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7-10% per U-Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands.

Metathesis reactions between uranium tetrachloride and lithium 2,6-diisopropylphenylamide in the presence of 4,4'-dialkyl-2,2'-bipyridyl (R(2)bpy; R = Me, (t)Bu) or triphenylphosphine oxide (tppo) appear to generate bis(imido)uranium(IV) in situ. These extremely reactive complexes abstract chloride from dichloromethane to generate U(NDipp)(2)Cl(R(2)bpy)(2) or U(NDipp)(2)Cl(tppo)(3) (Dipp = 2,6-(i)Pr(2)C(6)H(3)). The preparation of the bromide and iodide analogues U(NDipp)(2)X(R(2)bpy)(2) was achieved by addition of CH(2)X(2) (X = Br, I) to the uranium(IV) solutions. The uranium(V) halides were characterized by X-ray crystallography and found to exhibit linear N-U-N units and short U-N bonds. Electrochemical measurements were made on the chloride bipyridine species, which reacts readily with iodine or ferrocenium to generate bis(imido)uranium(VI) cations.

The conproportionation reaction between the dimeric diimidouranium(V) species [U(N(t)Bu)(2)(I)((t)Bu(2)bpy)](2) ((t)Bu(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridyl) and UI(3)(THF)(4) in the presence of additional (t)Bu(2)bpy yields U(N(t)Bu)(I)(2)((t)Bu(2)bpy)(THF)(2) (2), an unprecedented example of a monoimidouranium(IV) dihalide complex. The general synthesis of this family of uranium(IV) derivatives can be achieved more readily by adding 2 equiv of MN(H)R (M = Li, K; R = (t)Bu, 2,6-(i)PrC(6)H(3), 2-(t)BuC(6)H(4)) to UX(4) in the presence of coordinating Lewis bases to give complexes with the general formula U(NR)(X)(2)(L)(n) (X = Cl, I; L = (t)Bu(2)bpy, n = 1; L = THF, n = 2). The complexes were characterized by (1)H NMR spectroscopy and single-crystal X-ray diffraction analysis of compounds 2 and {U[N(2,6-(i)PrC(6)H(3))](Cl)(2)(THF)(2)}(2) (4). (The X-ray structures of 5 and 6 are reported in the Supporting Information.)

Being positive about anions: Hydrocarbon complexes containing negative‐valent Hf are obtained for the first time as tris(polyarene)hafnates(2−), polyarene=anthracene (An) and naphthalene, where the latter functions as a synthon for the unknown atomic Hf2− (see scheme, cot=1,3,5,7‐cyclooctatetraene). Tris(anthracene)metalates(2−) of Ti and Zr were also accessed, which completes an unprecedented triad of tris(arene)metal complexes.

Uranium tetrachloride undergoes facile reactions with 4,4'-dialkyl-2,2'-bipyridine, resulting in the generation of UCl4(R2bpy)2, R = Me, (t)Bu. These precursors, as well as the known UCl4(tppo)2 (tppo = triphenylphosphine oxide), react with 2 equiv of lithium 2,6-di-isopropylphenylamide to provide the versatile uranium(IV) imido complexes, U(NDipp)Cl2(L)n (L = R2bpy, n = 2; L = tppo, n = 3). Interestingly, U(NDipp)Cl2(R2bpy)2 can be used to generate the uranium(V) and uranium(VI) bisimido compounds, U(NDipp)2X(R2bpy)2, X = Cl, Br, I, and U(NDipp)2I2((t)Bu2bpy), which establishes these uranium(IV) precursors as potential intermediates in the syntheses of high-valent bis(imido) complexes from UCl4. The monoimido species also react with 4-methylmorpholine-N-oxide to yield uranium(VI) oxo-imido products, U(NDipp)(O)Cl2(L)n (L = (t)Bu2bpy, n = 1; L = tppo, n = 2). The aforementioned molecules have been characterized by a combination of NMR spectroscopy, X-ray crystallography, and elemental analysis. The chemical reactivity studies presented herein demonstrate that Lewis base adducts of uranium tetrachloride function as excellent sources of U(IV), U(V), and U(VI) imido species.

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High quality epitaxial thin films of cubic UC 2 were synthesized using a solution based technique. The films were characterized using XRD, UPS, Raman, and resistivity. The substrate lattice is yttrium stabilized zirconia and serves to stabilize the high temperature cubic phaseof UC 2 (>1765°C) at room temperature. The resistivity and UPS data indicate that UC 2 has relatively low electrical conductivity consistent with HSE hybrid DFT calculations showing a narrow band gap. In situ XRD measurements show that the UC 2 films oxidize to U 3 O 8 above 200°C. KEYWORDS: actinide carbide, epitaxial thin film, nuclear energy ■ INTRODUCTIONThe carbides of uranium have received much attention in recent years as potential fuel sources for Generation IV nuclear reactors. While at least six different Generation IV reactor concepts are currently being explored, the consensus is that new nuclear fuels must be developed for these advanced systems to maintain reasonable operating temperatures. 1 Two such possibilities are uranium carbide (UC) and uranium dicarbide (UC 2 ), which have much higher thermal conductivities than conventional nuclear fuels such as UO 2 , mixed-metal oxides, and ThO 2 . This would have the effect of lowering fuel centerline temperatures, which would increase operational safety, fuel lifetime, and efficiency. In addition to their potential as nuclear fuels, uranium carbide has been used for many years as a target material in the production of neutron-rich isotopes. 2 In order to gain a more in-depth understanding of the properties of uranium carbide materials, we have synthesized and characterized epitaxial thin films of UC 2 . These thin films are of high purity and are nearly single crystal in quality and thus facilitate high quality measurements to validate computational models. Herein, we report the synthesis and characterization of these thin films including valence band photoemission, Raman spectroscopy, electrical conductivity, and in situ XRD studies of oxidation.Uranium carbides have been known for quite some time, with U 2 C 3 and UC 2 first mentioned over a century ago. 3 These bulk materials were originally prepared by treating U 3 O 8 with graphite in an electric arc furnace. 3a,c,4 While carbothermic methods are still used to prepare uranium carbides, 5 other strategies such as the reaction of uranium metal with methane have been used to prepare UC at much lower temperature. 3b In spite of the wealth of knowledge on these materials, little is known about uranium carbide thin films. In 2004, researchers used sputter codeposition to generate films that were found to have compositions UC 1.2 and UC 1.6 . X-ray diffraction studies revealed the presence of polycrystalline UC and UC 2 in these films; however, the diffraction peaks were broadened possibly due to stress, inhomogeneities, or the presence of amorphous material. 6 Herein, we report the first preparation of singlecrystal quality UC 2 by polymer-assisted deposition (PAD). In this process, metal polymer solutions are used as film precursor...