Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U38. Issue 46 (29th September 2015)

• Record Type: Journal Article
• Authors: Falaise, Clément; Volkringer, Christophe; Hennig, Christoph; Loiseau, Thierry
• Is Part Of: Chemistry. Volume 21:Issue 46(2015)
• Page Start: 16654
• Page End: 16664
• Publication Date: 2015-09-29
• Subjects: EXAFS spectroscopy -- solvothermal synthesis -- uranium -- UV/Vis spectroscopy -- X‐ray diffraction; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201502207
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U38, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO2 from t =3 h, after the formation of unknown solid phases at an early stage. The crystallization of U38 occurred from t =4 h at the expense of UO2, and is completed after t =8 h. Starting from pure uranium(IV) species in solution ( t =0–1 h), oxidation reactions are observed with a U IV /U VI ratio of 70:30 for t =1–3 h. Then, the ratio is inversed with a U IV /U VI ratio of 25/75, when the precipitation of UO2 occurs. Thorough SEM observations of the U38 crystallites showed that the UO2 aggregates are embedded within. This may indicate that UO2 acts as reservoir of uranium(IV), for the formation of U38, stabilized by benzoate and THF ligands. During the early stages of the U38 crystallization, a transient crystallized phase appeared at t =4 h. Its crystal structure revealed a new dodecanuclear moiety (U12 ), based on the inner hexanuclear core of {U6 O8 } type, decorated by three additional pairs of dinuclear U2 units. The U12 motif is stabilized by benzoate, oxalates, and glycolate ligands. Abstract : Good for U38 : The formation of the large uranium(IV)‐based poly‐oxo cluster U38 has been studied by the combination of ex‐situ techniques. The results show the precipitation of a UO2 ‐fluorite‐like solid before the crystallization of the desired U38 compound. The latter is the thermodynamically stable phase in the presence of excess of benzoic acid and THF solvent.