Hydroxy- and alkoxy-bridged dinuclear uranyl–Schiff base complexes: hydrolysis, transamination and extraction studies

Abstract

The reaction of uranyl nitrate with 1,3-bis(salicylideneamino)-2-propanol (H₃L1) and 1,3-bis(3,5-di-tert-butylsalicylideneamino)-2-propanol (H₃L2) in the presence of triethylamine (Et₃N) yielded hydroxy- and alkoxy-bridged dinuclear complexes; [(UO₂)₂(L1)(OH)(MeOH)₂]·(MeOH)₂ (1·(MeOH)₂) and [(UO₂)₂(L2)(OH)(MeOH)₂]·(MeOH)₂ (2·(MeOH)₂). The crystal structures of 1·(DMF)₂ and 2·(DMF)₂ exhibit an unsymmetrical central U₂O₂ core involving bridging alkoxy- and hydroxy-oxygen atoms. The geometry around the uranium center in 1·(DMF)₂ and 2·(DMF)₂ is that of a distorted pentagonal bipyramid with the solvent molecule occupying the fifth coordination site. The flexible nature of the ligand backbone is more pronounced in 2·(DMF)₂ compared to 1·(DMF)₂, yielding two molecules per unit cell in different conformations. Under similar reaction conditions, using ethylenediamine as a base, the respective Salen-based uranyl compounds, [UO₂(Salen)(MeOH)] (3) and [UO₂(Bu^t₂-Salen)(MeOH)] (4) are obtained due to transamination of the ligand backbone. Complexes 1·(MeOH)₂ and 2·(MeOH)₂ when reacted with an excess of ethylenediamine failed to yield the respective Salen-based complexes, 3 and 4, respectively. The new compounds have been characterized using solution (NMR and UV-Vis) and solid-state (IR, X-ray crystallography) techniques. Hydrolysis of 1·(MeOH)₂ and 2·(MeOH)₂ in the pH range 1–14 was studied using UV-Vis spectroscopy and compared with the hydrolysis of 3 and [UO₂(Salophen)(MeOH)] (5). A two-phase extraction study suggests quantitative removal of uranyl ions from the aqueous phase at higher pH conditions.