The linear uranyl UO22+(VI) cation (D∞h symmetry) exhibited strong and broad absorptions at 350-400 nm in anhydrous methanol and methanol-water liquid mixtures in the UV-Vis spectra. The intensity of the absorptions (represented by absorbance at 375 nm) has been found to be directly proportional to molar concentration of methanol in the solutions. The absorptions at 350-400 nm originate from the charge-transfer (single-electron transfer) from CH3OH (electron donor) to UO22+(electron acceptor) within an initially formed [UO22+, CH3OH] electron-donor-acceptor (EDA) complex. Electron paramagnetic resonance (EPR) studies of various mixtures of UO22+-CH3OH and UO22+-CH3OH-H2O have shown that the charge-transfer also took place slowly in the dark, resulting in thermal reduction of UO22+(VI) to UO2+(V) (singlet, g=2.08) by CH3OH, and CH3OH in turn was oxidized to the hydroxymethyl .CH2OH radical (triplet, g = 2.01, aH = 21 G). The charge-transfer oxidation-reduction reaction is believed to take place via the EDA [UO22+, CH3OH] complex initially formed in the UO22+-CH3OH and UO22+-CH3OH-H2O mixtures. EPR studies indicated spin-spin coupling between UO2+(V) and .CH2OH in anhydrous methanol, serving as evidence for formation of the [UO2+, .CH2OH] ion-radical pair. The ion-radical pair was found to dissociate in the methanol-water mixtures. The EPR and UV-Vis studies support an inner-sphere charge-transfer mechanism for the uranyl(VI)-methanol reaction as shown below: UO22+ + CH3OH ≒ [UO22+, CH3OH] → [UO2+, .CH2OH] + H+ ≒ UO2+ + .CH2OH + H+ The EPR studies have also shown that UO22+(VI) was reduced to UO2+(V) thermally by diphenyl sulfide (Ph2S) and 2-methyl-5-(propan-2-yl)phenol (carvacrol, ArOH), respectively, in liquid acetone. Ph2S and ArOH were in turn oxidized to the diphenyl sulfide Ph2S+. radical (singlet, g=2.00) and carvacrol ArO. radical (singlet, g = 1.98), respectively. The reactions followed the charge-transfer pathway. EPR evidence indicated formation of the [UO2+, Ph2S+.] ion-radical pair in the charge-transfer reaction of UO22+ and Ph2S.

Thermal charge-transfer reduction of uranyl UO22+(VI) to UO2+(V) by methanol and other functionalized organic compounds

254th National Meeting (2017)

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