(VO)-O-IV and V-IV Species Formed in Aqueous Solution by the Tridentate Glutaroimide-Dioxime Ligand - an Instrumental and Computational Characterization

Complexation of V-IV in aqueous solution with glutaroimide-dioxime (H3L), a ligand proposed for the possible sequestration of uranium from seawater, was studied by the combined application of spectroscopic (EPR and UV/Vis), spectrometric (ESI-MS), electrochemical (CV), and computational (DFT) techniques. The results indicate that a rare non-oxido V-IV species, with formula [(VL2)-L-IV](2-), is formed in the pH range 3-5. It transforms into a usual (VO)-O-IV complex, [(VOL)-O-IV(OH)](2-), at pH > 6. The non-oxido species is characterized by a type 3 EPR spectrum with A(z) approximate to 126 x 10(-4) cm(-1) and a UV/Vis signal with epsilon > 2000 m(-1)cm(-1) in the visible region. The detection of V-V species by ESI-MS spectrometry was related to two possible oxidation processes, the first one in solution and the second one in-source during the recording of the spectra. The cyclic voltammogram of [(VL2)-L-IV](2-) shows two quasi-reversible processes, at E-1/2 = -0.75 V and E-1/2 = 0.03 V, assigned to the V-IV/V-III reduction and V-IV/V-V oxidation, respectively. All the experimental results were verified by DFT calculations, which indicated that the geometry of the non-oxido V-IV complex is intermediate between an octahedron and a trigonal prism and allowed us to predict its V-51 hyperfine coupling (HFC) tensor A, the electron absorption spectrum, the two redox processes in the cyclic voltammogram, and the electronic structure that, in turn, determines its EPR and UV/Vis behavior.