Organic ligands with carboxylate functionalities have been shown to affect the solubility, speciation, and overall chemical behavior of tetravalent metal ions. While many reports have focused on actinide complexation by relatively simple monocarboxylates such as amino acids, in this work we examined Th(IV) and U(IV) complexation by 4-hydroxybenzoic acid in water with the aim of understanding the impact that the organic backbone has on the solution and solid state structural chemistry of thorium(IV) and uranium(IV) complexes. Two compounds of the general formula [AnO(OH)(HO)(4-HB)]· nHO [An = Th (Th-1) and U (U-1); 4-HB = 4-hydroxybenzoate] were synthesized via room-temperature reactions of AnCl and 4-hydroxybenzoic acid in water. Solid state structures were determined by single-crystal X-ray diffraction, and the compounds were further characterized by Raman, infrared, and optical spectroscopies and thermogravimetry. The magnetism of U-1 was also examined. The structures of the Th and U compounds are isomorphous and are built from ligand-decorated oxo/hydroxo-bridged hexanuclear units. The relationship between the building units observed in the solid state structure of U-1 and those that exist in solution prior to crystallization as well as upon dissolution of U-1 in nonaqueous solvents was investigated using small-angle X-ray scattering, ultraviolet-visible optical spectroscopy, and dynamic light scattering. The evolution of U solution speciation as a function of reaction time and temperature was examined. Such effects as well as the impact of the ligand on the formation and evolution of hexanuclear U(IV) clusters to UO nanoparticles compared to prior reported monocarboxylate ligand systems are discussed. Unlike prior reported syntheses of Th and U(IV) hexamers where the pH was adjusted to ∼2 and 3, respectively, to drive hydrolysis, hexamer formation with the HB ligand appears to be promoted only by the ligand.

Uranium(IV)c omplexation by 2-furoic acid (2-FA) was examined to better understand the effects of ligand identity and reactionc onditions on speciesf ormationa nd stability.F ive compounds were isolated:[ UCl 2 (2-FA) 2 (H 2 O) 2 ] n (1), [U 4 Cl 10 O 2 (THF) 6 (2-FA) 2 ]·2THF (2), [U 6 O 4 (OH) 4 (H 2 O) 3 (2-FA) 12 ]·7THF·H 2 O( 3), [U 6 O 4 (OH) 4 (H 2 O) 2 (2-FA) 12 ]·8.76 H 2 O( 4), and [U 38 Cl 42 O 54 (OH) 2 (H 2 O) 20 ]·m H 2 O·n THF (5). The structures were determinedb ys ingle-crystal X-ray diffractiona nd further characterized by Raman, IR, and optical absorption spectroscopy.T he thermals tability and magnetic behavior of the compounds werealso examined. Variationsinthe synthetic conditions led to notable differences in the structural units observed in the solids tate. At low H 2 O/THF ratios, a tetranuclear oxo-bridged [U 4 O 2 ]c ore was isolated.A ging of this solution resulted in the formationaU 38 oxo cluster capped by chloro and water ligands. However,a ti ncreasing water concentrationso nly hexanuclear units wereo bserved. In all cases, at temperatures of 100-120 8C, UO 2 nanoparticles formed.[a] Dr.Supporting information (Structure refinement details;Thermal ellipsoid plots;B ond Valence Summation values;P owderX -ray diffractionpatterns; UV/Vis-NIR spectra;Raman and IR spectra;TGA spectra;M agnetic data; TEM data)a nd the ORCID identification number(s) for the author(s) of this articlecan be foundu nder: https://doi.Figure 5. Binding modes of the 2-FAu nits observed in 3 (a,c,d) and 4 (b-d).

A series of eighteen tetravalent actinide (An = Th, U, Pu) compounds were synthesized from acidic aqueous solutions containing thorium, uranium, or plutonium and a series of protonated nitrogen heterocycles. The compounds were characterized using Raman, IR, and optical absorption spectroscopies. The structures were determined using single-crystal X-ray diffraction and found to consist of [An(H 2 O) x Cl y ] 4−y (x = 4−7 and y = 2−4) or AnCl 6 2− molecular units. Breaks in the structural chemistry of the early actinides were observed, with Th adopting exclusively Th−aquo−chloro species and Pu forming only PuCl 62− ; U crystallized as both U−aquo−chloro and UCl 6 2− . The relationship between the solid-state structural units and the solution species was interrogated using UV−vis−near-IR absorption spectroscopy. A comparison of the solution and solid-state spectra suggested that, although prevalent in the solid state, particularly for U and Pu, AnCl 6 2− does not exist to an appreciable extent in the reaction solution. Despite the identification of U−aquo−chloro species in solution, there are limited reports of these complexes in the solid state. Isolation of these unique actinide(IV) chlorides as reported in this work may point to the importance of nonbonding interactions in the stabilization and precipitation of An IV structural units.

Aqueous reactions of thorium chloride and furanmono‐ and dicarboxylate ligands including 2‐furoate (2FA), 3‐furoate (3FA), and 2,5‐furandicarboxylate (2,5FDC) yielded four Th(IV) phases that exhibit diverse structural chemistry ranging from isolated molecular units to 3D topologies. [Th(2FA)4]n (Th‐1) and [Th(3FA)4]n (Th‐2) consist of isostructural ligand bridged 1D chains. The compounds exhibit wide stability, forming at temperatures of 20–120 °C and pH ca. 1–4. Using the bifunctional ligand 2,5FDC a stark difference is observed; Th2(2,5FDC)4(H2O)10·2H2O (Th‐3) is built from relatively rare ligand‐bridged molecular complexes. Th‐3 similarly exhibits broad synthetic stability, forming over 20–100 °C and pH 1.8–6. At higher temperatures; however, [Th(2,5FDC)2(H2O)2]n (Th‐4), which adopts an extended 3D network is observed. The structures were determined by single‐crystal X‐ray diffraction and further characterized via Raman and IR spectroscopy as well as thermogravimetric analysis. Overall, the work highlights the role of organic ligands in the stabilization of unique Th structural units.