Synthesis, X-ray structure, electrochemical behaviour and fluorescence studies of mononuclear dioxouranium complexes with oxygen donor ligands

Two phenol-based mononuclear dioxouranium complexes [UO2(mdbp)2{OHCH2CMe3}] (1) and [UO2(bdbp)2{OCHNMe2}] (2) (where mdbpH = 4-methyl-2,6-dibenzoylphenol and bdbpH = 4-tert-butyl-2,6-dibenzoylphendol) have been synthesized and characterized by mass spectrometry, IR, NMR and electronic spectroscopy, cyclic voltammetry and single crystal X-ray diffraction studies. In both the complexes the UVI atom has pentagonal bipyramidal geometry with two apical oxo groups, two chelating dibenzoyl phenolate ligands and one unidentate neopentanol or DMF molecule in the equatorial plane. The electrochemical behaviour of the complexes was investigated by cyclic voltammetry in CH3CN and DMSO. Both complexes exhibited a single quasireversible/irreversible reduction corresponding to the UO22+/UO21+ couple. The potential for 1 in DMSO shifted towards a more positive value by 0.604 V, indicating the exchange of coordinated neopentanol with DMSO. The fluorescence of 1 and 2 was studied by fluorometric method and was compared with that observed for the ligand precursors. The results indicated characteristic charge transfer emissions.

Graphical abstractThe reaction of monomeric bidentate ligands [(2,6-PhCO)2(4-R)C6H2OH] (R = Me or t-Bu) with [UO2(NO3)2·6H2O] gave uranyl complexes with distorted pentagonal bipyramidal geometry at the metal centre. The U–Oeq bond lengths increase in the order: phenolic < solvent < carbonyl oxygen. The bidentate OO-donor mdbpH/bdbpH ligands are thus good chelators for the UO22+ ion. The reduction potential of 2 with a tert-butyl substituent on the phenolate ring was shifted to a more negative potential than that of 1 with a methyl substituent, reflecting the stronger electron donating ability of the tert-butyl group. The coordinated neopentanol in 1 is readily displaced by DMSO solution. Both complexes exhibit characteristic charge transfer emissions.Figure optionsDownload full-size imageDownload as PowerPoint slide