V(IV)O 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 [V(IV)L2]2-, is formed in the pH range 3-5. It transforms into a usual V(IV)O complex, [V(IV)OL(OH)]2-, at pH > 6. The non-oxido species is characterized by a "type 3" EPR spectrum with Az ? 126 × 10-4 cm-1 and a UV/Vis signal with ? > 2000 m-1 cm-1 in the visible region. The detection of VV 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 [V(IV)L2]2- shows two quasi-reversible processes, at E1/2 = -0.75 V and E1/2 = 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 51V 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.