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

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 (UO 2 )(O 2 )·4H 2 O. All the compounds contain anionic uranyl polyhedra clusters with charge compensated by ammonium ions. Three are uranyl peroxides built from uranyl hexagonal bipyramids [(UO 2 )(O 2 ) 3 ] 4– or [(UO 2 )(O 2 ) 2 (OH) 2 ] 4– sharing peroxide or dihydroxyl equatorial edges to form cage clusters [(UO 2 ) 28 (O 2 ) 42 ] 28– (U 28 ) and [(UO 2 ) 44 (O 2 ) 66 ] 44– (U 44 ) in 7 and 3 respectively, or crown-shaped cluster [(UO 2 ) 32 (O 2 (OH) 2 ) 52 ] 40– (U 32R ) 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 [(UO 2 )(O 2 )(C 2 O 4 ) 2 ] bipyramids by peroxide ion sharing to form the dimer [(UO 2 ) 2 (O 2 )(C 2 O 4 ) 4 ] 6– (U 2 Ox 4 ) in 8, (ii) assembly of five [(UO 2 )(O 2 ) 2 (C 2 O 4 )] bipyramids linked by sharing peroxide to form the [(UO 2 ) 5 (O 2 ) 5 (C 2 O 4 ) 5 ] 10– (U 5 Ox 5 ) pentameric rings in 4 and 6, (iii) further condensation of 12 U 5 Ox 5 rings through bis-bidentate oxalates to create the [(UO 2)60 (O 2 ) 60 (C 2 O 4 ) 30 ] 60– (U 60 Ox 30 ) nanosphere in 5, (iv) replacing an oxalate by two hydroxide ions in U 5 Ox 5 rings and sharing of the OH–OH bridge between two pentamers to form the dimer of pentamers [(UO 2 ) 10 (O 2 ) 10 (OH) 2 (C 2 O 4 ) 8 ] 18– (U 10 Ox 8 ) 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.