Di-μ-acetato Diuranyl(VI) Complexes with N-(2-pyridyl)-N′-(5-R-salicylidene)hydrazines: Syntheses, Structures, Properties and Extraction Studies

Abstract

Reactions of equimolar amounts of UO2(OAc)2⋅2H2O and N-(2-pyridyl)-N′-(5-R-salicylidene)hydrazines (HLn; n=1–3 for R = H, Me, and OMe, respectively) in methanol produced [(UO2)2(μ-OAc)2(Ln)2] (1-3 for n=1–3) in ∼70% yields. Elemental analysis, various spectroscopic, and electrochemical measurements were used to characterize 1-3. X-ray crystal structures revealed an 8-membered U(OCO)2U ring with a chair conformation and pentagonal-bipyramidal UN2O5 coordination spheres in each of 1-3. The N,N,O-donor (Ln)- and the two O-atoms of the two bridging (OAc)- form the pentagonal plane around the U-atom of trans-(UO2)2+. Intermolecular H-bonding assisted self-assembly of the complexes lead to 1- and 2-dimensional supramolecular structures. Infrared (IR) and proton nuclear magnetic resonance (1H NMR) spectra of 1-3 are consistent with their molecular structures. The electronic spectra showed four strong absorption bands in the range 466-288 nm. The complexes undergo two irreversible reductions near -0.65 and -0.88 V (vs. Ag/AgCl). Time-dependent density functional theory (TD-DFT) calculations suggest that a ligand centered transition causes the highest energy absorption band, while, the remaining three are ligand-to-metal charge transfer (LMCT) bands and the two successive reductions are due to U(VI)-U(VI) → U(V)-U(VI) → U(V)-U(V) processes. A maximum extraction of 98% of trans-(UO2)2+ from the aqueous layer was observed in two-phase extraction studies.