Why Is Uranyl Formohydroxamate Red?

Abstract

The complexation of UO2(2+) by formohydroxamate (FHA(-)) creates solutions with dark red coloration. The inherent redox activity of formohydroxamate leads to the possibility that these solutions contain U(V) complexes, which are often red. We demonstrate that the reaction of U(VI) with formohydroxamate does not result in reduction, but rather in formation of the putative cis-aquo UO2(FHA)2(H2O)2, whose polymeric solid-state structure, UO2(FHA)2, contains an unusually bent UO2(2+) unit and a highly distorted coordination environment around a U(VI) cation in general. The bending of the uranyl cation results from unusually strong π donation from the FHA(-) ligands into the 6d and 5f orbitals of the U(VI) cation. The alteration of the bonding in the uranyl unit drastically changes its electronic and vibrational features.

Bio-inspired nano-traps for uranium extraction from seawater and recovery from nuclear waste

“…All three materials exhibited a dramatic color change upon exposure to uranium solutions, turning from white or light brown to orange-red (Supplementary Fig. 8 ) 42 . The equilibrium adsorption data were well fitted with the Langmuir model, yielding correlation coefficients higher than 0.99 (Fig.…”

Understanding the origins of Oyl–U–Oylbending in the uranyl (UO22+) ion

“…The bending of uranyl has been explored computationally by several different groups. 15,[41][42][43][44][45] In all cases, the trans isomer of uranyl was found to be more stable than the cis isomer. Interestingly, however, the relative energy difference of the trans and cis isomers appears to be strongly dependent on the identity of the equatorial co-ligands.…”

“…For example, the cis isomer of [UO2(OH)4] 2is 19 kcal/mol higher in energy than trans isomer, 42 whereas the cis isomer of [UO2(κ 2 -NO3)2(H2O)2] is 33 kcal/mol higher in energy than trans isomer. 45 Indeed, the lowest trans/cis isomerization energies are found for [UO2(OH)4] 2and [UO2(H2O)2(κ 2 -ONHCH=O)2], 42,43,45 which both feature strongly donating, anionic equatorial co-ligands. This observation is perhaps not surprising, as strong equatorial donors are known to weaken the U-Oyl bonds.…”

Effects of preorganization in the chelation of UO22+by hydroxamate ligands: cyclic PIPO−vs.linear NMA−

“…The hydrogen-bonded chain-like assembly of [UO2(NMA)2(H2O)] and [UO2(PIPO)2(H2O)] is original among the structures of uranyl bishydroxamato complexes deposited in the Cambridge Structural Database (CSD, release 5.38), 30 although formo-(FHAH), 31 aceto-(AHAH), 32 and salicyl-(SHAH) 33 hydroxamic acids (Chart 1) also form linear coordination polymers. However, in the latter cases no water molecule is involved in the assembly, since symmetry-related adjacent hexacoordinated uranyl cations (UU = 4.346-4.465 Å) are directly interacting with a pair of monodentate carbonyl oxygen donors and two 2-bridging bidentate hydroxamic N-O atoms.…”