To examine properties of pentavalent uranium, U(V), we have carried out electrochemical and spectroelectrochemical studies on UO2 (saloph)L [saloph = N,N′-disalicylidene-o-phenylenediaminate, L = dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF)]. The electrochemical reactions of UO2(saloph)L U02(saloph)L complexes in L were found to occur quasireversibly. The reduction processes of UO2(saloph)L complexes were followed spectroelectrochemically by using an optical transparent thin layer electrode cell. It was found that the absorption spectra measured at the applied potentials from 0 to -1.650 V versus ferrocene/ferrocenium ion redox couple (Fc/Fc+) for UO2(saloph)-DMSO in DMSO have clear isosbestic points and that the evaluated electron stoichiometry equals 1.08. These results indicate that the reduction product of UO2(saloph)DMSO is [UvO2(saloph)DMSO]-, which is considerably stable in DMSO. Furthermore, it was clarified that the absorption spectrum of the [UVO2(saloph)DMSO]- complex has a very small molar absorptivity in the visible region and characteristic absorption bands due to the 5f1 orbital at around 750 and 900 nm. For UO2(saloph)DMF in DMF, the clear isosbestic points were not observed in the similar spectral changes. It is proposed that the UO2(saloph)DMF complex is reduced to [UvO2(saloph)DMF]- accompanied by the dissociation of DMF as a successive reaction. The formal redox potentials of UO2(saloph)L in L (E0, vs Fc/Fc+) for U(VI)/U(V) couple were determined to be -1.550 V for L = DMSO and -1.626 V for L = DMF.
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