ABSTRACT
To clarify the electronic spectral properties of uranyl(V) complexes systematically, we measured absorption spectra of three types of pure uranyl(V) complexes: [U(V)O2(dbm)2DMSO]-, [U(V)O2(saloph)DMSO]-, and [U(V)O2(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, epsilon = 150-900 M(-1).cm(-1)) for [U(V)O2(dbm)2DMSO]-, 650, 750, 900, 1400, and 1875 nm (epsilon = 100-300 M(-1).cm(-1)) for [U(V)O2(saloph)DMSO]-, and 760, 990, 1140, 1600, and 1800 nm (epsilon = 0.2-3.6 M(-1).cm(-1)) for [U(V)O2(CO3)3]5-. These characteristic absorption bands of the uranyl(V) complexes are attributable to the electronic transitions in the U(V)O2+ core because the spectral features are similar to each other despite the differences in the ligands coordinated to the equatorial plane of the U(V)O2+ moiety. On the other hand, the epsilon values of [U(V)O2(CO3)3]5- are quite smaller than those of [U(V)O2(dbm)2DMSO]- and [U(V)O2(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.
ABSTRACT
We have measured 13C NMR spectra of uranyl(V) carbonate complex in D2O solution containing 1.003 M Na2(13)CO3 at various temperatures. Two singlet signals corresponding to free and coordinated CO3(2-) were observed at 169.13 and 106.70 ppm, respectively. From the peak area ratio, the structure of the uranyl(V) carbonate complex was determined as [U(V)O2(CO3)3]5-. Furthermore, kinetic analyses of the exchange reaction of free and coordinated CO3(2-) in [U(V)O2(CO3)3]5- were carried out using 13C NMR line-broadening. As a result, the first-order rate constant at 298 K and the activation parameters for CO3(2-) exchange reaction in [U(V)O2(CO3)3]5- were evaluated as 1.13 x 10(3) s(-1) and deltaH(double dagger) = 62.0 +/- 0.7 kJ x mol(-1), deltaS(double dagger) = 22 +/- 3 J x mol(-1) x K(-1), respectively. We suggest that the exchange follows a dissociative mechanism as in the corresponding [U(VI)O2(CO3)3]4- complex.
ABSTRACT
We have obtained the first evidence that the U[double bond]O bond strength in uranyl moiety is weakened with the reduction from U(VI)O(2)(2+) to U(V)O(2)(+) from the IR spectroelectrochemical study on U(VI)O(2)(saloph)DMSO and [U(V)O(2)(saloph)DMSO](-) (saloph = N,N'-disalicylidene-o-phenylenediaminate, DMSO = dimethyl sulfoxide) complexes with the thin layer electrode cell for IR measurements.
ABSTRACT
To examine properties of pentavalent uranium, U(V), we have carried out electrochemical and spectroelectrochemical studies on UO(2)(saloph)L [saloph = N,N'-disalicylidene-o-phenylenediaminate, L = dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF)]. The electrochemical reactions of UO(2)(saloph)L complexes in L were found to occur quasireversibly. The reduction processes of UO(2)(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 UO(2)(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 UO(2)(saloph)DMSO is [U(V)O(2)(saloph)DMSO](-), which is considerably stable in DMSO. Furthermore, it was clarified that the absorption spectrum of the [U(V)O(2)(saloph)DMSO](-) complex has a very small molar absorptivity in the visible region and characteristic absorption bands due to the 5f(1) orbital at around 750 and 900 nm. For UO(2)(saloph)DMF in DMF, the clear isosbestic points were not observed in the similar spectral changes. It is proposed that the UO(2)(saloph)DMF complex is reduced to [U(V)O(2)(saloph)DMF](-) accompanied by the dissociation of DMF as a successive reaction. The formal redox potentials of UO(2)(saloph)L in L (E(0), 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.
