Direct evidence of uranium(V) intermediates by electron spin resonance in photo- and electrolytic reduction processes of uranyl complexes in organic solutions

ESR of pentavalent uranium has been observed in both processes of photo- and electrolytic-reduction of UO2(dimethylformamide)5(ClO4)2 in dimethylformamide. The ESR spectra are nearly symmetric. This means the case of g-factor in an axial symmetry with g⊥ > g∥. Taking the values of g⊥ = 2.5 observed and g∥ ∼ 0 and a usual value of 1900 cm−1 of spin-orbit coupling constant for uranium(V), the result of ESR is analyzed along with the experimental values of electronic spectra from visible to near infrared. Based on crystalline field theory, U(V) species in the present solution have ligands held more tightly in the equatorial plane than in the axial direction and not a form of UO2+, as expected.