Spectroscopic Characterization of L-ascorbic Acid-induced Reduction of Vanadium(V) Dipicolinates: Formation of Vanadium(III) and Vanadium(IV) Complexes from Vanadium(V) Dipicolinate Derivatives

A Br-substituted dioxidovanadate dipicolinate complex was prepared and structurally characterized by X-ray crystallography. The counter ion in the solid state is the ammonium cation in a monoclinic crystal cell with, P2(1)/c. The X-ray structure is similar to the structure generated from the analogous nitro substituted dipicolinate ligand. When exploring reactions of this and other dipicolinate-containing compounds with L-ascorbic acid, both vanadium(IV) and vanadium(III) species were formed under acidic conditions; then identified by conventional (X-band) and high-frequency and -field electron paramagnetic resonance (EPR) spectroscopy. A reaction with half an equivalent of L-ascorbic acid led to the formation of vanadium(IV) species, and addition of substituents (X = H, Cl, Br, OH, and NH2) in the para position of dipicolinic acid did not have a major effect on the vanadium(IV) species formed. In a reaction with excess L-ascorbic acid and vanadium(V)dipic-X complexes (where X = H, Cl, Br, OH, and NH2), vanadium(III) species were observed, thus proving that vanadium(III) species can be formed under acidic conditions resembling those in biological systems.