Uranyl(VI) complexes of [O,N,O,N′]-type diaminobis(phenolate) ligands: Syntheses, structures and extraction studies

The syntheses and crystal structures of four new uranyl complexes with [O,N,O,N′]-type ligands are described. The reaction between uranyl nitrate hexahydrate and the phenolic ligand [(N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylenediamine)], H2L1 in a 1:2 molar ratio (M to L), yields a uranyl complex with the formula [UO2(HL1)(NO3)] · CH3CN (1). In the presence of a base (triethylamine, one mole per ligand mole) with the same molar ratio, the uranyl complex [UO2(HL1)2] (2) is formed. The reaction between uranyl nitrate hexahydrate and the ligand [(N,N-bis(2-hydroxy-3,5-di-t-butylbenzyl)-N′,N′-dimethylethylenediamine)], H2L2, yields a uranyl complex with the formula [UO2(HL2)(NO3)] · 2CH3CN (3) and the ligand [N-(2-pyridylmethyl)-N,N-bis(2-hydroxy-3,5-dimethylbenzyl)amine], H2L3, in the presence of a base yields a uranyl complex with the formula [UO2(HL3)2] · 2CH3CN (4). The molecular structures of 1–4 were verified by X-ray crystallography. The complexes 1–4 are zwitter ions with a neutral net charge. Compounds 1 and 3 are rare neutral mononuclear [UO2(HLn)(NO3)] complexes with the nitrate bonded in η2-fashion to the uranyl ion. Furthermore, the ability of the ligands H2L1–H2L4 to extract the uranyl ion from water to dichloromethane, and the selectivity of extraction with ligands H2L1, H3L5 (N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-3-amino-1-propanol), H2L6 · HCl (N,N-bis(2-hydroxy-5-tert-butyl-3-methylbenzyl)-1-aminobutane · HCl) and H3L7 · HCl (N,N-bis(2-hydroxy-5-tert-butyl-3-methylbenzyl)-6-amino-1-hexanol · HCl) under varied chemical conditions were studied. As a result, the most efficient and selective ligand for uranyl ion extraction proved to be H3L7 · HCl.

Graphical abstractThe reaction between uranyl nitrate hexahydrate and phenolic ligand precursors N,N-bis(2-hydroxy-3,5-dialkylbenzyl)-N′,N′-dimethylethylenediamine (alkyl = methyl or t-butyl) (H2L1, H2L2) in a 1:2 molar ratio (M to L) in acetonitrile afford a uranyl complex of the form [UO2(HLn)(NO3)] · xCH3CN (n = 1, x = 1 and n = 2, x = 2) whereas H2L1 and N-(2-pyridylmethyl)-N,N-bis(2-hydroxy-3,5-dimethylbenzyl)amine (H2L3) in the presence of a base (triethylamine) afford the mononuclear, neutral uranyl complexes [UO2(HLn)2] · xCH3CN (n = 1, x = 0 and n = 3, x = 2). Extraction studies of the uranyl ion from water to dichloromethane with these ligands are also presented.Figure optionsDownload full-size imageDownload as PowerPoint slide