Layered and Three-Dimensional Framework Cesium and Barium Uranyl Carboxyphenylphosphonates

The hydrothermal reactions of either uranyl nitrate or uranium trioxide with 4-carboxyphenylphosphonic acid and HF/CsOH in the presence of Cs+ or Ba2+ cations result in the formation of Cs2{(UO2)2(PO3HC6H4CO2H)3(PO3C6H4CO2H)F} (CsUcpp-1), Cs4{(UO2)6(PO3C6H4CO2H)8}(H2O)7 (CsUcpp-2), Ba{(UO2)2(PO3C6H4CO2H)2F2}(H2O)2 (BaUcpp-1), and Cs3{(UO2)(O3PC6H4CO2)}3(H2O)3·4H2O (CsUcpp-3). All these compounds contain the UO22+ moiety coordinated by five additional donor atoms resulting in a pentagonal bipyramidal coordination environment. CsUcpp-1, CsUcpp-2, and BaUcpp-1 were isolated at low pH, whereas CsUcpp-3 was synthesized at high pH giving rise to complete elimination of protonation of phosphonic and carboxylic acid moieties. The pH of the reaction is playing a vital role in the formation of the pillared structures in which the carboxylate and phosphonate moieties are coordinated to uranium center in CsUcpp-3, while in the other three compounds, the carboxylate moiety coordinate exclusively to the Cs+ or Ba2+ cations.