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, [MoVI(O)(η2-NH2O)]3+, specifically [MoVI(O)(η1,η1,η1-pidiox-O,N,O')(η2-NH2O)(H2O)], formed via hydrolysis of (2Z,6Z)-piperidine-2,6-dione dioxime (H3pidiox) by Mo(VI). Τhis discovery enabled us to elucidate the long-standing mechanism of Mo(VI) conversion to MoII-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 [MoII(η1,η1,η1-Hpidiox-O,N,O')(κ1-NO)(η2-NH2O)(OH2)]. In parallel, we show that H3pidiox, a ligand employed in uranyl extraction from seawater, undergoes hydrolysis by [MoVIO4]2- only upon formation of the complex cis-[MoVIO2(pidiox)]+. However, this complex is hydrolytically unstable at pH 8.0, suggesting that [MoVIO4]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.