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)­(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.