Complexation of U(VI) with benzoic acid at variable temperatures (298-353 K): thermodynamics and crystal structures of U(VI)/benzoate complexes

摘要

Thermodynamics of the U(VI) complexation with benzoic acid (HL) was studied by spectrophotometry at varied temperatures (298-353 K) with constant ionic strength (1.05 mol kg(-1) NaClO4). Two U(VI) benzoate complexes, UO2L+ and UO2(OH)L(aq), were identified and their formation constants determined. The formation of both complexes is endothermic and driven exclusively by entropy. Two types of U(VI)/benzoate complex crystals were synthesized from aqueous solutions at different pH and ligand/metal ratios. Their structures were determined by single-crystal X-ray diffractometry. One structure is a 1 : 3 U(VI) benzoate complex (Na[UO2(C7H5O2)(3)]center dot 2H(2)O), each benzoate holding a bidentate coordination mode to U(VI) in the equatorial plane of UO22+. The other is a U(VI) hydroxobenzoate complex with unique mu(3)-OH bridging ([(UO2)(2)(C7H5O2)(2)(mu(3)-OH)(2)]center dot 4H(2)O). In the structure, each UO22+ ion holds five coordination oxygens in its equatorial plane, two carboxylate oxygens from two benzoate ligands and three oxygens from three mu(3)-OH groups.