Naturally occurring uranium is a widespread contaminant present in the water resources around the abandoned uranium mines in the southwest United States. A novel method for rapid uranium detection has been recently developed that relies on the sequestering of uranium by amidoximated polyacrylonitrile (AO-PAN) polymer mats and uses the Raman-active (ν1) symmetric stretch as the signal. The Raman signals obtained from uranium bearing AO-PAN were challenging to interpret due to an unknown uranyl speciation on the surface of the mats. Herein, we provide the synthesis and structural characterization of six model coordination compounds that contain acetamidoxime/benzamidoxime (AAO/BAO) coordinated to the uranyl cation: [UO2(η1-AAO)­(NO3)2(H2O)] (1), [UO2(η1-AAO)2(NO3)2] (2), [UO2(η2-BAO)2(CH3OH)2] (3), [(UO2)3(η2-BAO)3(μ2-NO3)3] (4), [(UO2)4(μ3-O)2(μ2-BAO)4(η1-BAO)4(H2O)2]­(NO3)4 (5), and [(UO2)4(μ3-O)2(μ2-BAO)4(η1-BAO)6Na­(NO3)2]­(NO3)3 (6). Solid-state Raman spectra of 1–6 showed dramatic differences in the uranyl ν1 symmetric stretch depending on the coordination of the amidoxime functional group. The assignments made from the solid-state Raman spectra were used to deconvolute the solution-state Raman spectra of uranyl–acetamidoxime/benzamidoxime methanol solutions at different metal to ligand molar ratios. At low molar ratios (1 U:1 AAO/BAO and 1 U:2 AAO/BAO) the dominant species is the uranyl coordinated via the η1-oxygen atom of the oxime group, while at high molar ratios (1 U:3 AAO/BAO and 1 U:4 AAO/BAO) the dominant species are a tetrameric uranyl−μ3-O-η1-amidoxime complex similar to compounds 5 and 6 and a uranyl−η2-amidoxime complex similar to compounds 3 and 4. Solid-state Raman spectra showed good agreement with Raman signals obtained from the uranyl–AO-PAN mats, demonstrating that binding motifs between uranyl and amidoxime in compounds 5 and 6 are the most representative of the uranyl species on the surface of the AO-PAN mats.

The uranyl cation, (U(VI)O2)2+, has previously demonstrated photocatalytic reactivity in organic solutions, which results in the formation of uranyl peroxide species. Past studies indicated that the phototransformation process typically ends with the formation of uranyl peroxide solid phases. In the current study, we explore the transformation of uranyl nitrate crown ether complexes, (18‐crown‐6)[UO2(NO3)2(H2O)2] ⋅ 2 H2O (1 a), (18‐crown‐6)[UO2(NO3)2(H2O)2] (1 b), and (18‐crown‐6) [K(18‐crown‐6)] [(UO2)2(OH)2(NO3)4(H2O)4] (2) in the presence of ethanol to create a uranyl peroxide intermediate phase, (18‐crown‐6)[(UO2)2(O2)(NO3)2(H2O)4] (3), followed by a second alteration to a black solid (4). These compounds were structurally evaluated using both single‐crystal and powder X‐ray diffraction and then further characterized using Raman, NMR, and XPS spectroscopy. The initial transformation from the yellow uranyl nitrate phase (1 a, 1 b, and 2) to the uranyl peroxide (3) follow previously reported mechanisms. The second transformation to the black phase (4) is likely due to additional degradation of the 18‐crown‐6 molecule as a result of hydrogen abstraction and ring‐opening. In addition, we discuss the nature of confinement effects to impact the hydrogen abstraction process and the possibility that nitrate anions in the system may synergistically enhance the degradation process and lead to the formation of 4.