SCHEDULED MAINTENANCE
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.
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