Four dioxido vanadium(V) complexes with formula [VO2(L)]− (where L = Schiff base ligand, formed by reacting 3‑hydroxy-2-naphtoic hydrazide with 5-chlorosalicylaldehyde or 5-bromosalicylaldehyde) have been isolated and structurally characterized based on elemental analysis, IR and UV–Vis spectra results. The X-ray crystal structure diffraction analyses of all complexes indicate a distorted square pyramidal geometry for the vanadium(V) atom, being coordinated by the tridentate Schiff base and two oxido groups. The complexes are charge-balanced by triethylammonium or diethylammonium cations, and the hydrogen bond interactions between the ionic species are discussed. Ionic vanadium complexes are characterized by better solubility in water compared to neutral molecules, and this represents an important feature if these compounds have to be used as pharmaceuticals. The stability of the complexes at pH = 2 and 7 was established by UV–Vis spectroscopy and compared to the previously studied potential insulin-mimetic vanadium(III-V) complexes with Schiff base ligands. The effect of the cation on the solubility of the complexes in water and their stability in solution has been described in detail.
