Modeling Mo(VI)═O Biologically Related Interactions with Oximes and Hydroxylamines: Implications for Uranium Seawater Extraction

Molybdenum enzymes play a crucial role in the nitrogen cycle processes. However, the mechanism of Mo(VI) reduction by hydroxylamine/oximes and its implications for oxime-based sorbents remain unclear. For decades, it has been widely accepted that the reaction of NH₂OH with Mo(VI) consistently results in the molybdenum reduction. This study presents evidence that challenges the prevailing view by isolating the first Mo(VI)-oxido-hydroxylamido complex, [Mo^VI(O)(η²-NH₂O)]³⁺, specifically [Mo^VI(O)(η¹,η¹,η¹-pidiox-<i>O</i>,<i>N</i>,<i>O</i>')(η²-NH₂O)(H₂O)], formed via hydrolysis of (2<i>Z</i>,6<i>Z</i>)-piperidine-2,6-dione dioxime (H₃pidiox) by Mo(VI). Τhis discovery enabled us to elucidate the long-standing mechanism of Mo(VI) conversion to Mo^II-NO through a combination of experimental techniques (NMR, ESI-MS, XPS, FT-IR) and density functional theory (DFT) calculations. This comprehensive approach provided new insight into molybdenum redox behavior and unambiguously confirmed the Mo(II) oxidation state in [Mo^II(η¹,η¹,η¹-Hpidiox-<i>O</i>,<i>N</i>,<i>O</i>')(κ¹-NO)(η²-NH₂O)(OH₂)]. In parallel, we show that H₃pidiox, a ligand employed in uranyl extraction from seawater, undergoes hydrolysis by [Mo^VIO₄]^2- only upon formation of the complex <i>cis</i>-[Mo^VIO₂(pidiox)]⁺. However, this complex is hydrolytically unstable at pH 8.0, suggesting that [Mo^VIO₄]^2- is unlikely to degrade uranium oxime-based extraction materials in seawater. This study provides fundamental insight into molybdenum-oxime reactivity, offering a molecular basis for designing robust oxime-functionalized materials for efficient and durable uranium seawater extraction processes.