Coordination structures for uranyl carboxylate complexes in aqueous solution studied by IR and carbon-13 NMR spectra

Abstract

The interaction of uranyl ion with acetic, glycolic, malic, tartaric, tricarballylic, and citric acids in aqueous solution has been investigated by means of IR and 13C NMR spectroscopy. The complex formation is reflected in the IR frequency shifts for the COO-stretching vibrations (yM(COO) and vs(COO)) and the asymmetric 0=U=0 stretching mode 03). The possible coordination structures of COO groups to the uranyl ion in the individual systems are discussed in terms of the IR frequencies of the vas(COO) and ys(COO) modes. The model calculation of the nas(COO) and vs(COO) frequencies based on a normal-coordinate treatment has indicated that the carboxylate coordination in the uranyl acetate and tricarballylate complexes is assigned to a bidentate structure in which both the two oxygen atoms in a given COO group take part in the coordination and that in the uranyl glycolate, lactate, malate, tartrate, and citrate complexes only one of the two oxygen atoms in a given carboxylate of the individual ligands coordinates to the uranyl ion. The carboxylate 13C NMR signals for all the systems examined have experienced significant shifts going from the individual free ligands to the complex species, which suggests the coordination of the carboxylate groups to the uranyl ion. Consideration of the alcoholic 13C NMR signals of the uranyl complexes involving glycolate, lactate, malate, tartrate, and citrate has indicated that the uranyl ion interacts with the alcoholic oxygen as well as the carboxylate oxygen of the individual ligands. The presence of some uranyl complex species, in which the alcoholic oxygen in a given -COH group coordinates to uranyl ion with dissociation of protons, was revealed by the observation of an unusually large lower magnetic field shift of the alcoholic carbon signal. The possible structures of the main uranyl complex species present in aqueous solution, compatible with the IR and 13C NMR evidence, are proposed and discussed.

Variable denticity in carboxylate binding to the uranyl coordination complexes

“…For the UO 2 (OAc) 3 Ϫ in an organic solvent (CH 2 Cl 2 ), the asym and sym values were similar to those in aqueous media, at 1548 and 1461 cm…”

“…Bidentate coordination is also seen in the IR spectra of crystalline uranyl acetate compounds: the asym and sym values for Na[UO 2 (OAc) 3 ] were 1537 and 1472 cm Ϫ1 (⌬ asym-sym value of 65 cm Ϫ1 ), [5] and in this salt the binding mode was confirmed by the crystal structure [6]. The interpretation of the IR spectrum of crystalline UO 2 (OAc) 2 )) [2].…”

“…, and ⌬ asym-sym from 72 to 60 cm Ϫ1 [1][2][3][4]. However, additional asym and sym vibrational frequencies have been measured that range from 1595 to 1603 and 1389 to 1390 cm Ϫ1 (⌬ asym-sym ϭ 206 -213 cm…”

The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography

“…In the solution phase, as mentioned above, based on the concentration analysis and potentiometric studies previously reported [15,23] the only uranyl species is the 2:2 binuclear uranyl malate complex. It is expected that the stability constants of most of the complex species change with temperature.…”

“…Unfortunately only n 3 , the U = O asymmetric stretching, has been measured. It was found to be 962.4 cm -1 in the case of the uranyl-aqua ion [23] and to decrease to 916.2 cm -1 in the case of U(VI) malate [17,23]. This reduction in the energy of n 3 , which is accompanied by bond elongation in case of the U(VI) malate species, is believed to be due to the stronger coordination of the U atom by the oxygen atoms of the malate ligand.…”