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

Abstract

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]·2H₂O), 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 μ₃-OH bridging ([(UO₂)₂(C₇H₅O₂)₂(μ₃-OH)₂]·4H₂O). 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 μ₃-OH groups.