Lucile Chatelain

A Uranium-Based UO2+-Mn2+ Single-Chain Magnet Assembled trough Cation-Cation Interactions

Single-chain magnets (SCMs) are materials composed of magnetically isolated one-dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide-containing SCM. The 5f-3d heterometallic 1D chains [{[UO2(salen)(py)][M(py)4](NO3)}]n, (M=Cd (1) and M=Mn (2); py=pyridine) are assembled trough cation-cation interaction from the reaction of the uranyl(V) complex [UO2(salen)py][Cp*2Co] (Cp*=pentamethylcyclopentadienyl) with Cd(NO3)2 or Mn(NO3)2 in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm(-1)), probably as a result of strong intra-chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K.

Self-assembly of a 3d-5f trinuclear single-molecule magnet from a pentavalent uranyl complex.

Mixed-metal uranium compounds are very attractive candidates in the design of single-molecule magnets (SMMs), but only one 3d-5f hetero-polymetallic SMM containing a uranium center is known. Herein, we report two trimeric heterodimetallic 3d-5f complexes self-assembled by cation-cation interactions between a uranyl(V) complex and a TPA-capped M(II)  complex (M=Mn (1), Cd (2); TPA=tris(2-pyridylmethyl)amine). The metal centers were strategically chosen to promote the formation of discrete molecules rather than extended chains. Compound 1, which contains an almost linear {MnOUOMn} core, exhibits SMM behavior with a relaxation barrier of 81±0.5 K-the highest reported for a mono-uranium system-arising from intramolecular Mn-U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. Compound 1 also exhibits an open magnetic hysteresis loop at temperatures less than 3 K, with a significant coercive field of 1.9 T at 1.8 K.

Synthesis and Structure of Nitride-Bridged Uranium(III) Complexes

ABSTRACT The reduction of the nitride-bridged diuranium(IV) complex Cs[{U(OSi(OtBu)3)3}2(μ-N)]affords the first example of a uranium nitride complex containing uranium in the +III oxidation state. Two nitride-bridged complexes containing the heterometallic fragments Cs2[U(III---)-N-(---U(IV)] and Cs3[U(III---)-N-(---U(III)] have been crystallographically characterized. The presence of two or three Cs+ cations binding the nitride group is key for the isolation of these complexes. In spite of the fact that the nitride group is multiply bound to two uranium and two or three Cs+ cations, these complexes transfer the nitride group to CS2 to afford SCN(-) and uranium(IV) disulfide.

Single-molecule-magnet behavior in mononuclear homoleptic tetrahedral uranium(III) complexes.

The magnetic properties of the two uranium coordination compounds, [K(18c6)][U(OSi(O(t)Bu)3)4] and [K(18c6)][U(N(SiMe3)2)4], both presenting the U(III) ion in similar pseudotetrahedral coordination environments but with different O- or N-donor ligands, have been measured. The static magnetic susceptibility measurements and density functional theory studies suggest the presence of different ligand fields in the two compounds. Alternating-current susceptibility studies conducted at frequencies ranging from 95 to 9995 Hz and at temperatures in the 1.7-10 K range revealed for both compounds slow magnetic relaxation already at zero static magnetic field with similar energy barriers U ∼24 K.

Nitrogen reduction and functionalization by a multimetallic uranium nitride complex

Molecular nitrogen (N2) 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 N2 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 N2 under mild conditions are known, with co-operativity between metal centres considered crucial for the N2 reduction step, the multimetallic species involved are usually not well defined, and further transformation of N2-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 N2. However, few examples of uranium complexes binding N2 are known, and soluble uranium complexes capable of transforming N2 into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N2 under ambient conditions by a fully characterized complex with two Uiii ions and three K+ centres held together by a nitride group and a flexible metalloligand framework. The addition of H2 and/or protons, or CO to the resulting complex results in the complete cleavage of N2 with concomitant N2 functionalization through N–H or N–C bond-forming reactions. These observations establish that a molecular uranium complex can promote the stoichiometric transformation of N2 into NH3 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 N2 under mild conditions.

Ferrocene-Based Tetradentate Schiff Bases as Supporting Ligands in Uranium Chemistry

Uranyl(VI), uranyl(V), and uranium(IV) complexes supported by ferrocene-based tetradentate Schiff-base ligands were synthesized, and their solid-state and solution structures were determined. The redox properties of all complexes were investigated by cyclic voltammetry. The bulky salfen-(t)Bu2 allows the preparation of a stable uranyl(V) complex, while a stable U(IV) bis-ligand complex is obtained from the salt metathesis reaction between [UI4(OEt2)2] and K2salfen. The reduction of the [U(salfen)2] complex leads to an unprecedented intramolecular reductive coupling of the Schiff-base ligand resulting in a C-C bond between the two ferrocene-bound imino groups.

Nucleophilic Reactivity of a Nitride-Bridged Diuranium(IV) Complex: CO2 and CS2 Functionalization

Thermolysis of the nitride-bridged diuranium(IV) complex Cs{(μ-N)[U(OSi(O(t) Bu)3)3]2} (1) showed that the bridging nitride behaves as a strong nucleophile, promoting N-C bond formation by siloxide ligand fragmentation to yield an imido-bridged siloxide/silanediolate diuranium(IV) complex, Cs{(μ-N(t) Bu)(μ-O2 Si(O(t) Bu)2)U2 (OSi(O(t) Bu)3)5}. Complex 1 displayed reactivity towards CS2 and CO2 at room temperature that is unprecedented in f-element chemistry, affording diverse N-functionalized products depending on the reaction stoichiometry. The reaction of 1 with two equivalents of CS2 yielded the thiocyanate/thiocarbonate complex Cs{(μ-NCS)(μ-CS3 )[U(OSi(O(t)Bu)3)3]2} via a putative NCS(-)/S(2-) intermediate. The reaction of 1 with one equivalent of CO2 resulted in deoxygenation and N-C bond formation, yielding the cyanate/oxo complex Cs{(μ-NCO)(μ-O)[U(OSi(O(t) Bu)3 )3]2}. Addition of excess CO2 to 1 led to the unprecedented dicarbamate product Cs{(μ-NC2O4)[U(OSi(O(t) Bu)3)3]2}.

Heterometallic Fe2II-UV and Ni2II-UV Exchange-coupled Single-Molecule Magnets: Effect of the 3d Ion on the Magnetic Properties

Uranium-based compounds have been put forward as ideal candidates for the design of single-molecule magnets (SMMs) with improved properties, but to date, only two examples of exchange-coupled 3d-5f SMM containing uranium have been reported and both are based on the Mn(II) ion. Here we have synthesized the first examples of exchange-coupled uranium SMMs based on Fe(II) and Ni(II) . The SMM behavior of these complexes containing a quasi linear {M-O=U=O-M} core arises from intramolecular Fe-U and Ni-U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. The measured values of the relaxation barrier (53.9±0.9 K in the UFe2 complex and of 27.4±0.5 K in the UNi2 complex) show clearly the dependency on the spin value of the transition metal, providing important new information for the future design of improved uranium-based SMMs.

Isolation of a Star-Shaped Uranium(V/VI) Cluster from the Anaerobic Photochemical Reduction of Uranyl(VI)

Actinide oxo clusters are an important class of compounds due to their impact on actinide migration in the environment. The photolytic reduction of uranyl(VI) has potential application in catalysis and spent nuclear fuel reprocessing, but the intermediate species involved in this reduction have not yet been elucidated. Here we show that the photolysis of partially hydrated uranyl(VI) in anaerobic conditions leads to the reduction of uranyl(VI), and to the incorporation of the resulting UV species into the stable mixed-valent star-shaped UVI /UV oxo cluster [U(UO2 )5 (μ3 -O)5 (PhCOO)5 (Py)7 ] (1). This cluster is only the second example of a UVI /UV cluster and the first one associating uranyl groups to a non-uranyl(V) center. The UV center in 1 is stable, while the reaction of uranyl(V) iodide with potassium benzoate leads to immediate disproportionation and formation of the U12IV U4V O24 cluster {[K(Py)2 ]2 [K(Py)]2 [U16 O24 (PhCOO)24 (Py)2 ]} (5).

CO Cleavage and CO2 Functionalization under Mild Conditions by a Multimetallic CsU2 Nitride Complex

Novel efficient chemical processes involving cheap and widely accessible carbon dioxide or carbon monoxide under mild conditions for the production of valuable chemical products are highly desirable in the current energetic context. Uranium nitride materials act as high activity catalysts in the Haber-Bosch process but the reactivity of molecular nitride compounds remains unexplored. Here we review recent results obtained in our group showing that a multimetallic nitride complex [Cs{[U(OSi(OtBu)3)3]2(μ-N)}] (1) with a CsUIV-N-UIV core, is able to promote N-C bond formation due to its strong nucleophile behaviour. In particular, complex 1, in the presence of excess CO2 leads to a remarkable dicarbamate product. The multimetallic CsUIV-N-UIV nitride also readily cleaves the C≡O bond under mild conditions.