Uranyl Speciation on the Surface of Amidoximated Polyacrylonitrile Mats

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.