Role of Ammonium Ions in the Formation of Ammonium Uranyl Peroxides and Uranyl Peroxo-oxalates

Abstract

The flexibility of the ammonium ion environment is suitable for inducing the formation of several uranyl peroxides and peroxo-oxalates. For a given concentration of uranium and various oxalate/uranium ratios, by varying the pH with ammonium hydroxide, crystals of eight compounds have been isolated and characterized by X-ray diffraction, in addition to the studtite (UO2)(O2)·4H2O. All the compounds contain anionic uranyl polyhedra clusters with charge compensated by ammonium ions. Three are uranyl peroxides built from uranyl hexagonal bipyramids [(UO2)(O2)3]4– or [(UO2)(O2)2(OH)2]4– sharing peroxide or dihydroxyl equatorial edges to form cage clusters [(UO2)28(O2)42]28– (U28) and [(UO2)44(O2)66]44– (U44) in 7 and 3 respectively, or crown-shaped cluster [(UO2)32(O2(OH)2)52]40– (U32R) in 2. In these uranyl peroxides the pentagonal and hexagonal uranyl polyhedra rings are stabilized by ammonium ions. The other five compounds are uranyl peroxo-oxalates with various condensations of uranyl hexagonal bipyramids: (i) condensation of two [(UO2)(O2)(C2O4)2] bipyramids by peroxide ion sharing to form the dimer [(UO2)2(O2)(C2O4)4]6– (U2Ox4) in 8, (ii) assembly of five [(UO2)(O2)2(C2O4)] bipyramids linked by sharing peroxide to form the [(UO2)5(O2)5(C2O4)5]10– (U5Ox5) pentameric rings in 4 and 6, (iii) further condensation of 12 U5Ox5 rings through bis-bidentate oxalates to create the [(UO2)60(O2)60(C2O4)30]60– (U60Ox30) nanosphere in 5, (iv) replacing an oxalate by two hydroxide ions in U5Ox5 rings and sharing of the OH–OH bridge between two pentamers to form the dimer of pentamers [(UO2)10(O2)10(OH)2(C2O4)8]18– (U10Ox8) in 9. In the last four compounds, the ammonium ions stabilize the pentameric cycles. The ammonium ion has different effects, particularly in the case of pentamers, and thus provides access to a large panel of cluster sizes.