Inorganica Chimica Acta p. 12 - 20 (2018)
Update date:2022-08-29
Topics:
Sun, Xiaoping
Kolling, Derrick R.J.
Deskins, Seth
Adkins, Ethan
The linear uranyl UO22+(VI) cation (D∞h symmetry) exhibited strong and broad absorptions at 350–400 nm in anhydrous methanol and methanol-water mixtures in the UV-Vis spectra. The intensity of the absorptions (represented by absorbance at 375 nm) is directly proportional to molar concentrations of methanol and UO22+(VI), respectively. The linear relationships indicate formation of an electron-donor-acceptor (EDA) complex [UO22+, CH3OH]. The absorptions at 350–400 nm originate from the charge-transfer (single-electron transfer) from CH3OH (electron donor) to UO22+ (electron acceptor) within the [UO22+, CH3OH] 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 was oxidized to the hydroxymethyl [rad]CH2OH radical (generating an axial signal). The charge-transfer oxidation-reduction reaction is believed to take place via the EDA [UO22+, CH3OH] complex. EPR studies suggested spin-spin coupling between UO2+(V) and [rad]CH2OH in anhydrous methanol, supporting the formation of a [UO2+, [rad]CH2OH] ion-radical pair. The EPR studies have also shown that UO22+(VI) was reduced to UO2+(V) thermally by other alcohols (ethanol, 2-propanol, and cyclohexanol), and by diphenyl sulfide (Ph2S), L-ascorbic acid (AA), and 2-methyl-5-(propan-2-yl)phenol (carvacrol, ArOH), respectively. Ph2S, AA, and ArOH were oxidized to the diphenyl sulfide Ph2S+[rad] radical cation (singlet, g = 2.00), ascorbic acid AA[rad] radical (singlet, g = 2.00), and carvacrol ArO[rad] radical (singlet, g = 1.98), respectively. Both EPR and UV-Vis studies indicate that the reactions followed the ground-state charge-transfer mechanisms similar to that of the UO22+/methanol reaction. EPR evidence supported formation of the [UO2+, Ph2S+[rad]] ion-radical pair in the charge-transfer reaction of UO22+ and Ph2S and spin-spin interactions within the ion-radical pair. The sulfuric-acid-catalyzed isomerization of [rad]CH2OH to CH3O[rad] was found by EPR studies.
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