Investigation of charge-transfer absorptions in the uranyl UO22+(VI) ion and related chemical reduction of UO22+(VI) to UO2+(V) by UV–Vis and electron paramagnetic resonance spectroscopies

•Charge-transfer absorptions of UO22+(VI) in Br−, I−, PhOH, and DMSO were observed.•Reaction of UO22+(VI) with X− (X = Br or I) gave UO2+(V) and a ion-radical pair X2−.•Reaction of UO22+(VI) with PhOH led to UO2+(V) and the PhO radical.•Reaction of UO22+(VI) with CH3SOCH3 gave UO2+(V) and the CH3SOCH2 radical.

The uranyl UO22+(VI) cation (hydrated) exhibited strong charge-transfer absorptions at 350–400 nm in aqueous solutions containing bromide and iodide. The charge-transfer absorptions originate from a single-electron transfer from a halide anion to the uranium(VI) valence shell. Their intensities (represented by absorbance at 375 nm) were found to be directly proportional to molar concentrations of the halide (bromide or iodide) and UO22+ in solution, respectively, showing the nature of a bimolecular interaction in the charge-transfer absorption transition. The absorptions were also greatly enhanced by sulfuric acid, and their intensity (absorbance at 375 nm) increased linearly as a function of the acid molarity. An electron paramagnetic resonance (EPR) study has indicated that the charge-transfer also took place slowly in the dark, resulting in appreciable thermal chemical reduction of diamagnetic UO22+(VI) (hydrated) to paramagnetic UO2+(V) (hydrated) (g = 2.08) by bromide and iodide. In the presence of sulfuric acid, CH3SOCH3 (DMSO) was shown by EPR to undergo a charge-transfer oxidation by UO22+(VI) to a stable CH3SOCH2 (DMSO) radical (singlet, g = 2.01), and UO22+(VI) was reduced to UO2+(V). A possible mechanism for this oxidation–reduction has been proposed. The charge-transfer absorption transition (350–400 nm) between UO22+(VI) and phenol (PhOH) in acetone was observed and characterized. A chemical oxidation–reduction of UO22+(VI) [in the form of UVIO2(acetone)52+] and PhOH in acetone was found by EPR to give UO2+(V) [in the form of UVO2(acetone)5+] and a stable phenoxyl (PhO) radical (singlet, g = 2.00) via a simultaneous charge-transfer and deprotonation pathway.

Graphical abstractCharge-transfer oxidation–reduction between UO22+(VI) and phenol (PhOH) leads to give UO2+(V) and the phenoxyl PhO radical.Figure optionsDownload full-size imageDownload as PowerPoint slide