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Uranium plays an unquestionable role in the framework of nuclear physics, biology, and radiopharmacy. Moreover, uranyl ion UO22+ offers an immense variety of applications due to the unique photosensitivity of its complexes. The excited state of uranyl cation is indeed accessible under ultraviolet‐visible (UV–vis) light, readily producing radical species UO22+* upon light irradiation. Herein, an innovative synthesis protocol is presented to explore the use of uranyl cations as photocatalyst systems for photocurable sol–gel‐based formulations, coupling the photochemical reactions of uranyl cations with photopolymerization‐based additive manufacturing processes. Additive manufacturing has nowadays revolutionized the production of complex structures with arbitrary geometries and has opened up enticing opportunities for innovative technological breakthroughs and highly tailorable systems. The fabrication of micro‐architected components is shown via vat photopolymerization, namely, the Digital Light Processing technique, and ‐3D) printed parts are converted into uranium dicarbide (UC2)/carbon nanocomposite upon carbothermal reduction. This uranyl‐mediated additive manufacturing process constitutes the first application of the synergistic role of uranyl motifs in a photopolymer platform, demonstrating for the first time the possibility to directly pattern uranium‐based materials in complex structures. [ABSTRACT FROM AUTHOR]

The hydrothermal decomposition of actinide(IV) oxalates (An= Th, U, Pu) at temperatures between 95 and 250 °C is shown to lead to the production of highly crystalline, reactive actinide oxide nanocrystals (NCs). This aqueous process proved to be quantitative, reproducible and fast (depending on temperature). The NCs obtained were characterised by X-ray diffraction and TEM showing their size to be smaller than 15 nm. Attempts to extend this general approach towards transition metal or lanthanide oxalates failed in the 95–250 °C temperature range. The hydrothermal decomposition of actinide oxalates is therefore a clean, flexible and powerful approach towards NCs of AnO2 with possible scale-up potential.

In recent years, the hydrothermal conversion of actinide (IV) oxalates into nanometric actinide dioxides (AnO2) has begun to be investigated as an alternative to the widely implemented thermal decomposition method. We present here a comparison between the hydrothermal and the conventional thermal decomposition of Pu(IV) oxalate in terms of particle size, morphology and residual carbon content. A parametric study was carried out in order to define the temperature and time applied in the hydrothermal conversion of tetravalent Pu-oxalate into PuO2 and to optimize the reaction conditions. [ABSTRACT FROM AUTHOR]

This paper details the first dedicated production of homogeneous nanocrystalline particles of mixed actinide oxide solid solutions containing americium. The target compositions were U0.75Pu0.20Am0.05O2, U0.90Am0.10O2 and U0.80Am0.20O2. After successful hydrothermal synthesis and chemical characterisation, the nanocrystals were sintered and their structure and behaviour under self-irradiation were studied by powder XRD. Cationic charge distribution of the as-prepared nanocrystalline and sintered U0.80Am0.20O2 materials was investigated applying U M4 and Am M5 edge high energy resolution XANES (HR-XANES). Typical oxidation states detected for the cations are U(IV)/U(V) and Am(III)/Am(IV). The measured crystallographic swelling was systematically smaller for the as-synthesised nanoparticles than the sintered products. For sintered pellets, the maximal volumetric swelling was about 0.8% at saturation, in line with literature data for PuO2, AmO2, (U,Pu)O2 or (U,Am)O2. [ABSTRACT FROM AUTHOR]

The structure and bonding properties of 16 complexes formed by trivalent f elements (M=U, Np, Pu and lanthanides except for Pm and Pr) with cyclopentadienyl (Cp) and cyclohexylisonitrile (C≡NCy) ligands, (Cp)3M(C≡NCy), were studied by a joint experimental (XRD, NMR) and theoretical (DFT) analysis. For the large La(III) ion, the bis-adduct (Cp)3La(C≡NCy)2 could also be synthesized and characterized. The metal–ligand interactions, focusing on the comparison of the actinides and lanthanides as well as on the competition of the two different ligands for M, were elucidated using the Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) models. The results point to interactions of comparable strengths with the anionic Cp and neutral C≡NCy ligands in the complexes. The structural and bonding properties of the actinide complexes reflect small but characteristic differences with respect to the lanthanide analogues. They include larger ligand-to-metal charge transfers as well as metal–ligand electron-sharing interactions. The most significant experimental marker of these covalent interactions is the C≡N stretching frequency. [ABSTRACT FROM AUTHOR]

A series of tetradentate N2O2-type Schiff base complexes with tetravalent 4f- and 5f-block metals, [M(salpn)2] (H2salpn = N,N′-disalicylidene-1,3-diaminopropane; M = Ce, Th, U, Np, and Pu), were prepared to systematically investigate their solid state structure, and their complexation behaviour in solution with the goal to investigate the subtle differences between 4f- and 5f-elements. X-ray diffraction revealed that all investigated metal cations form [M(salpn)2] complexes. All the complexes show the same ligand arrangement with meridional conformation, amongst which only Ce(IV) exhibits unique behaviour upon crystallisation. [Ce(salpn)2] crystallises in two less symmetric systems (P1¯ or P21/n), whilst all the other [M(salpn)2] crystallise in a more symmetric orthorhombic system (Pban). Quantum chemical calculations suggest that the observed structural peculiarity of Ce(IV) stems from the geometrical flexibility due to the more "ionic" nature of bonds to the 4f element. 1H NMR measurements revealed that [M(salpn)2] forms two different species in solution with and without an additional solvent molecule, where the relative distribution of the two species depends mainly on the ionic radius of the metal centre. Again, Ce(IV) behaves differently from the tetravalent actinides with a higher ratio of the solvent-molecule-coordinated species than the ratio expected from its ionic radius. Hence, this study is successful in observing subtle differences between 4f- (i.e. Ce) and 5f-elements (actinides; Th, U, Np, and Pu) both in the solid state and in solution on an analytically distinguishable level, and in relating the observed subtle differences to their electronic structure. [ABSTRACT FROM AUTHOR]

The synthesis of three complex series of the form [AnCl2(salen)(Pyx)2] (H2salen=N,N′‐bis(salicylidene)ethylenediamine; Pyx=pyridine, 4‐methylpyridine, 3,5‐dimethylpyridine) with tetravalent early actinides (An=Th, U, Np, Pu) is reported with the goal to elucidate the affinity of these heavy elements for small neutral N‐donor molecules. Structure determination by single‐crystal XRD and characterization of bulk powders with infrared spectroscopy reveals isostructurality within each respective series and the same complex conformation in all reported structures. Although the trend of interatomic distances for An−Cl and An−N (imine nitrogen of salen or pyridyl nitrogen of Pyx) was found to reflect an ionic behavior, the trend of the An−O distances can only be described with additional covalent interactions for all elements heavier than thorium. All experimental results are supported by quantum chemical calculations, which confirm the mostly ionic character in the An−N and An−Cl bonds, as well as the highest degree of covalency of the An−O bonds. Structurally, the calculations indicate just minor electronic or steric effects of the additional Pyx substituents on the complex properties. [ABSTRACT FROM AUTHOR]

The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1‐pyrazolyl)borato (Tp) ligand An[η3‐HB(N2C3H3)3]3 (AnTp3) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models. [ABSTRACT FROM AUTHOR]