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 (UO2)(O-2)center dot 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 bip-yramids [(UO2)(O-2)(3)](4-) or [(UO)(2)(O-2)(2)(OH)(2)](4-) (OH)2]4 - sharing peroxide or dihydroxyl equatorial edges to form cage clusters [(UO2)(28)(O-2)(42)](28-) (U-28) and [(UO2)(44)(O-2)(66)](44-) (U-44) in 7 and 3 respectively, or crown-shaped cluster [(UO2)(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 [(UO2)(O-2)(C2O4)(2)] bipyramids by peroxide ion sharing to form the dimer [(UO2)(2)(O-2)(C2O4)(4)](6-) (U(2)Ox(4)) in 8, (ii) assembly of five [(UO2)(O-2)(2)(C2O4)] bipyramids linked by sharing peroxide to form the [(UO2)(5)(O-2)(5)(C2O4)(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 [(UO2)(60)(O-2)(60)(C2O4)(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 [(UO2)(10)(O-2)(10)(OH)(2)(C2O4)(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.