Novel Dinuclear Uranyl Complexes with Asymmetric Schiff Base Ligands:  Synthesis, Structural Characterization, Reactivity, and Extraction Studies

Abstract

The reaction of uranyl nitrate with asymmetric [3O, N] Schiff base ligands in the presence of base yields dinuclear uranyl complexes, [UO2(HL1)]2.DMF (1), [UO2(HL2)]2.2DMF.H2O (2), and [UO2(HL3)]2.2DMF (3) with 3-(2-hydroxybenzylideneamino)propane-1,2-diol (H3L1), 4-((2,3-dihydroxypropylimino)methyl)benzene-1,3-diol (H3L2), and 3-(3,5-di-tert-butyl-2-hydroxybenzylideneamino)propane-1,2-diol (H3L3), respectively. All complexes exhibit a symmetric U2O2 core featuring a distorted pentagonal bipyramidal geometry around each uranyl center. The hydroxyl groups on the ligands are attached to the uranyl ion in chelating, bridging, and coordinate covalent bonds. Distortion in the backbone is more pronounced in 1, where the phenyl groups are on the same side of the planar U2O2 core. The phenyl groups are present on the opposite side of U2O2 core in 2 and 3 due to electronic and steric effects. A similar hydrogen-bonding pattern is observed in the solid-state structures of 1 and 3 with terminal hydroxyl groups and DMF molecules, resulting in discrete molecules. Free aryl hydroxyl groups and water molecules in 2 give rise to a two-dimensional network with water molecules in the channels of an extended corrugated sheet structure. Compound 1 in the presence of excess Ag(NO3) yields {[(UO2)(NO3)(C6H4OCOO)](NH(CH2CH3)3)}2 (4), where the geometry around the uranyl center is hexagonal bipyrimidal. Two-phase extraction studies of uranium from aqueous media employing H3L3 indicate 99% reduction of uranyl ion at higher pH.