Structural and spectroscopic studies of a rare non-oxido V(v) complex crystallized from aqueous solution

A non-oxido V(v) complex with glutaroimide-dioxime (H₃L), a ligand for recovering uranium from seawater, was synthesized from aqueous solution as Na[V(L)₂]·2H₂O, and the structure determined by X-ray diffraction. It is the first non-oxido V(v) complex that has been directly synthesized in and crystallized from aqueous solution. The distorted octahedral structure contains two fully deprotonated ligands (L^3-) coordinating to V⁵⁺, each in a tridentate mode via the imide N (R _V-N = 1.96 Å) and oxime O atoms (R _V-O = 1.87-1.90 Å). Using ¹⁷O-labelled vanadate as the starting material, concurrent ¹⁷O/⁵¹V/¹H/¹³C NMR, in conjunction with ESI-MS, unprecedentedly demonstrated the stepwise displacement of the oxido V[double bond, length as m-dash]O bonds by glutaroimide-dioxime and verified the existence of the "bare" V⁵⁺/glutaroimide-dioxime complex, [V(L)₂]^-, in aqueous solution. In addition, the crystal structure of an intermediate 1 : 1 V(v)/glutaroimide-dioxime complex, [VO₂(HL)]^-, in which the oxido bonds of vanadate are only partially displaced, corroborates the observations by NMR and ESI-MS. Results from this work provide important insights into the strong sorption of vanadium on poly(amidoxime) sorbents in the recovery of uranium from seawater. Also, because vanadium plays important roles in biological systems, the syntheses of the oxido and non-oxido V⁵⁺ complexes and the unprecedented demonstration of the displacement of the oxido V[double bond, length as m-dash]O bonds help with the on-going efforts to develop new vanadium compounds that could be of importance in biological applications.