The Photochemical Behavior of the Uranyl Ion (UO$\rm{{_{2}^{2+}}}$): a Dimer Perspective and Outlook

Abstract

Dedicated to Jean-Claude Bünzli on the occasion of his 65th birthdayThe complex formation of uranyl (UO 2þ 2 ) with oxalic acid (HOOCÀCOOH) in acetone is studied by UV/VIS, absorption, luminescence, and excitation spectroscopy. Based on solid-state crystallographic data, we propose a dimer structure with D 2h symmetry for the complex in solution. This symmetry is vibrationally distorted to D 2 by the out-of-plane equatorial-ligand vibration. From the spectroscopic point of view, this vibration induces intensity in the transitions P g S þ g and one component ofFrom the photochemical point of view, this vibration induces a twisting mechanism that destroys the complex. From the theoretical point of view, it is worthwhile to notice that the symmetry of the odd outof-plane vibration is the same as the symmetry of the odd LUMO (f xyz ). By vibrating accordingly to the LUMO symmetry, the complex is self-destroying by absorption of light, and the uranyl is regenerated. A small comment is devoted to a possible d -d interaction and the quintuple U 2 bond distance proposed by Gagliardi and Ross [29].

Structure and Dynamics of Uranyl(VI) and Plutonyl(VI) Cations in Ionic Liquid/Water Mixtures via Molecular Dynamics Simulations

“…87−89 The well-studied hydrolysis of uranyl shows dimers and trimers with bridging OH ligands existing in solution along with monomers. 90−98 The separation between the uranium atoms in these dimers is generally 0.38−0.49 nm, 85,88,89,91,92,94,95,98 which is considerably shorter than the distance observed here, indicating that Tf 2 N is a much weaker coordinating ligand. The exception is perrhenate-bridged dimers, which have U−U distances of 0.66−0.69 nm.…”

“…The uranyl perrhenate dimeric complex in [BMIM][Tf 2 N] was not observed in UV–vis and EXAFS experiments, although the solutions used in the experiments had a very dilute concentration of uranyl (0.01–0.001 M) . Experiments have shown that perrhenate and oxalate ligands can bridge to form uranyl dimeric complexes under the correct conditions. − Carboxylate and carbonate ligands can also cause dimerization. − The well-studied hydrolysis of uranyl shows dimers and trimers with bridging OH ligands existing in solution along with monomers. − …”

Aqueous coordination chemistry and photochemistry of uranyl(vi) oxalate revisited: a density functional theory study

“…From the solid state structures, 37 it is expected that the most probable structure is two chelate + one unidentate binding. 20 We have studied three possible structures which are listed in Table 1 (S15-S17). From the Gibbs energy obtained by the DFT calculations, the most stable structure is S15.…”

“…Our results are consistent with the previous work by G örller-Walrand and Servaes who proposed a photochemical reaction mechanism involving a 2 : 5 complex, although their study was performed in acetone and not in aqueous solution. 20 Fig. 4 The structure of the lowest-lying triplet state of uranyl(VI) oxalate 2 : 5 complex (T15).…”

“…19 In a very recent UV-Vis absorption and luminescence spectroscopic study in acetone, G örller-Walrand and Servaes proposed a photochemical reaction involving a uranyl(VI) oxalate dimer (2 : 5) complex. 20 They suggest that the out-of-plane bending of the bridging oxalate induces intramolecular twisting which may then destroy the oxalate ligand itself. A spectrophotometric study by Havel et al 2 demonstrated that such dimeric species may also prevail in the aqueous phase and their inclusion slightly improves the overall fitting of the equilibrium constants.…”