Comparison of Thorium(IV) and Uranium(VI) Carboxyphosphonates

The hydrothermal reactions of thorium nitrate and uranyl acetate with carboxyphenylphosphonic acid and HF result in the formation of ThF2(PO3C6H4CO2H) and UO2(PO3HC6H4CO2H)2·2H2O, respectively. ThF2(PO3C6H4CO2H) adopts a pillared structure constructed from thorium oxyfluoride layers built from [ThO4F4] units that are bridged by carboxyphenylphosphonate to yield a three-dimensional network. UO2(PO3HC6H4CO2H)2·2H2O forms one-dimensional chains of UO6 tetragonal bipyramids that are bridged by the phosphonate moiety of the ligand. The carboxylate portion of the structure links the chains together into layers via a hydrogen-bonding network. The higher effective charge and more isotropic coordination of the Th(IV) centers versus the uranium centers contained within uranyl cations allow for coordination by the protonated carboxylate portions of the ligands to the thorium cations, which does not occur with uranium in carboxyphosphonates. UO2(PO3HC6H4CO2H)2·2H2O displays vibronically coupled fluorescence and the potential for energy transfer from the ligand to the charge-transfer bands of the uranyl cations. The main fluorescence by the ligand is quenched in ThF2(PO3C6H4CO2H).