Electronic Spectra of Pure Uranyl(V) Complexes:  Characteristic Absorption Bands Due to a U^VO₂⁺ Core in Visible and Near-Infrared Regions¹

To clarify the electronic spectral properties of uranyl(V) complexes systematically, we measured absorption spectra of three types of pure uranyl(V) complexes:  [U^VO₂(dbm)₂DMSO]^-, [U^VO₂(saloph)DMSO]^-, and [U^VO₂(CO₃)₃]^5- (dbm = dibenzoylmethanate, saloph = N,N‘-disalicylidene-o-phenylenediaminate, DMSO = dimethyl sulfoxide). As a result, it was found that these uranyl(V) complexes have characteristic absorption bands in the visible−near-infrared (NIR) region, i.e., at around 640, 740, 860, 1470, and 1890 nm (molar absorptivity, ε = 150−900 M^-1·cm^-1) for [U^VO₂(dbm)₂DMSO]^-, 650, 750, 900, 1400, and 1875 nm (ε = 100−300 M^-1·cm^-1) for [U^VO₂(saloph)DMSO]^-, and 760, 990, 1140, 1600, and 1800 nm (ε = 0.2−3.6 M^-1·cm^-1) for [U^VO₂(CO₃)₃]^5-. These characteristic absorption bands of the uranyl(V) complexes are attributable to the electronic transitions in the U^VO₂⁺ core because the spectral features are similar to each other despite the differences in the ligands coordinated to the equatorial plane of the U^VO₂⁺ moiety. On the other hand, the ε values of [U^VO₂(CO₃)₃]^5- are quite smaller than those of [U^VO₂(dbm)₂DMSO]^- and [U^VO₂(saloph)DMSO]^-. Such differences can be explained by the different coordination geometries around the center uranium in these uranyl(V) complexes. Consequently, the absorption bands of the uranyl(V) complexes in visible−NIR region were assigned to f−f transitions in the 5f¹ configuration.