Uranyl Ion Complexation by Citric and Citramalic Acids in the Presence of Diamines

Uranyl nitrate reacts with citric (H4cit) or d-(−)-citramalic (H3citml) acids under mild hydrothermal conditions and in
the presence of diamines to give different complexes which are all characterized by the presence of 2:2 uranyl/polycarboxylate dianionic dimers or of polymeric chains based on the same dimeric motif. Each uranium ion is
chelated by the two ligands through the alkoxide and the α- or β-carboxylate groups, the second β-carboxylic
group in citrate being uncoordinated. The uranium coordination sphere is completed by either a water molecule or
the β-carboxylate group of a neighboring unit, thus giving zero- or one-dimensional assemblages, respectively.
The evidence for [UO2(Hcit)]2 dimers in the solid state confirms previous results from potentiometric and EXAFS
measurements on solutions. Depending on the diamine used (DABCO, 2,2‘- and 4,4‘-bipyridine, [2.2.2]cryptand)
and its ability to form divergent hydrogen bonds or not, different uranyl/polycarboxylate topologies are obtained,
thus evidencing template effects, and extended hydrogen bonding gives two- or three-dimensional assemblages.
These results, together with those previously obtained with NaOH as a base, add to the knowledge of the uranyl/citrate system, which is much investigated for its environmental relevance.