No abstract

Molecular nitrogen (N) is cheap and widely available, but its unreactive nature is a challenge when attempting to functionalize it under mild conditions with other widely available substrates (such as carbon monoxide, CO) to produce value-added compounds. Biological N fixation can do this, but the industrial Haber-Bosch process for ammonia production operates under harsh conditions (450 degrees Celsius and 300 bar), even though both processes are thought to involve multimetallic catalytic sites. And although molecular complexes capable of binding and even reducing N under mild conditions are known, with co-operativity between metal centres considered crucial for the N reduction step, the multimetallic species involved are usually not well defined, and further transformation of N-binding complexes to achieve N-H or N-C bond formation is rare. Haber noted, before an iron-based catalyst was adopted for the industrial Haber-Bosch process, that uranium and uranium nitride materials are very effective heterogeneous catalysts for ammonia production from N. However, few examples of uranium complexes binding N are known, and soluble uranium complexes capable of transforming N into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N under ambient conditions by a fully characterized complex with two U ions and three K centres held together by a nitride group and a flexible metalloligand framework. The addition of H and/or protons, or CO to the resulting complex results in the complete cleavage of N with concomitant N functionalization through N-H or N-C bond-forming reactions. These observations establish that a molecular uranium complex can promote the stoichiometric transformation of N into NH or cyanate, and that a flexible, electron-rich, multimetallic, nitride-bridged core unit is a promising starting point for the design of molecular complexes capable of cleaving and functionalizing N under mild conditions.

Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+...K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+...UO2+ and UO2+...K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+...K+ interaction plays an important synergistic role in the stabilization of the UO2+...UO2+ interactions. Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+...UO2+ interaction. The UO24+...UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.

Discrete molecular compounds that exhibit both magnetization hysteresis and slow magnetic relaxation below a characteristic 'blocking' temperature are known as single-molecule magnets. These are promising for applications including memory devices and quantum computing, but require higher spin-inversion barriers and hysteresis temperatures than currently achieved. After twenty years of research confined to the d-block transition metals, scientists are moving to the f-block to generate these properties. We have now prepared, by cation-promoted self-assembly, a large 5f-3d U(12)Mn(6) cluster that adopts a wheel topology and exhibits single-molecule magnet behaviour. This uranium-based molecular wheel shows an open magnetic hysteresis loop at low temperature, with a non-zero coercive field (below 4 K) and quantum tunnelling steps (below 2.5 K), which suggests that uranium might indeed provide a route to magnetic storage devices. This molecule also represents an interesting model for actinide nanoparticles occurring in the environment and in spent fuel separation cycles.

Stable complexes of pentavalent uranyl [UO(2)(salan-(t)Bu(2))(py)K](n) (3), [UO(2)(salan-(t)Bu(2))(py)K(18C6)] (4), and [UO(2)(salophen-(t)Bu(2))(thf)]K(thf)(2)}(n) (8) have been synthesized from the reaction of the complex {[UO(2)py(5)][KI(2)py(2)]}(n) (1) with the bulky amine-phenolate ligand potassium salt K(2)(salan-(t)Bu(2)) or the Schiff base ligand potassium salt K(2)(salophen-(t)Bu(2)) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [U(V)O(2)(salan-(t)Bu(2))(py)][Cp*(2)Co], 6, were also isolated from the reduction of [U(VI)O(2)(salan-(t)Bu(2))(py)], 5, with Cp*(2)Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO(2)py(5)]I.py (2). The measured redox potentials E(1/2) (U(VI)/U(V)) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give mu(eff) of 2.20 and 2.23 mu(B) at 300 K respectively, which is compared with a mu(eff) of 2.6(1) mu(B) (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g(x) = 1.98; g(y) = 1.25; g(z) = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the pi(u)*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO(2)(salan-(t)Bu(2))(py)K] and [UO(2)(salophen-(t)Bu(2))(py)K] complexes with respect to disproportionation and hydrolysis.

Siloxides can support U! …︁in the reduction of small molecules with uranium complexes. The treatment of [U{N(SiMe3)2}3] with HOSi(OtBu)3 (3 equiv) yielded a novel homoleptic uranium(III) siloxide complex 1, which acted as a two‐electron reducing agent toward CS2 and CO2 (see scheme). Complex 1 also reduced toluene to afford a diuranium inverted‐sandwich complex.

The new homoleptic ate U(III) siloxide [K(18c6)][U(OSi(O(t)Bu)3)4] 2 was prepared in 69% yield by reduction of [U(OSi(O(t)Bu)3)4] 3 with KC8. The reaction of the neutral U(III) siloxide complex [U(OSi(O(t)Bu)3)2(μ-OSi(O(t)Bu)3)]2 1 with adamantyl azide leads to the isolation of the dinuclear U(VI) imido complex [U2(NAd)4(OSi(O(t)Bu)3)4] 4. The X-ray crystal structure shows the presence of a "cation-cation interaction" between the two [U(NAd)2](2+) groups. In contrast the reactions of 2 with the trimethylsilyl and adamantyl azides afford the U(V) imido complexes [K(18c6)][U(NSiMe3)(OSi(O(t)Bu)3)4] 5-TMS and [K(18c6)][U(NAd)(OSi(O(t)Bu)3)4] 5-Ad pure in 48% and 66% yield, respectively. The reaction of 2 with CsN3 in THF at -40 °C yields a mixture of products from which the azido U(IV) complex [K(18c6)][U(N3)(OSi(O(t)Bu)3)4] 7 and the μ-nitrido diuranium(V) complex [KU(μ-N)(OSi(O(t)Bu)3)]2 8 were isolated. The crystal structure of 8 shows the presence of a rare U2N2 core with two nitrido atoms bridging two uranium centers in a diamond-shaped geometry. In contrast, the reaction of 1 with CsN3 affords the diuranium(IV) complex Cs{(μ-N)[U(OSi(O(t)Bu)3)3]2} 9 presenting a nitrido ligand bridging two uranium and one cesium cations. These results show the importance of the coordination environment in the outcome of the reaction of U(III) with azides.

The polymeric complex {[UO2Py5][KI2Py2]}n was isolated by controlled oxidation of uranium tris-iodide in pyridine and structurally characterized using X-ray diffraction. The described synthetic method allows us to isolate a stable derivative of the elusive pentavalent UO2+ species providing a potential starting material for the development of anhydrous UO2+ coordination chemistry.