Vanadium(IV)-citrate complex interconversions in aqueous solutions. A pH-dependent synthetic, structural, spectroscopic, and magnetic study.

Citrate is abundantly encountered in biological fluids as a natural metal ion chelator. Vanadium participates in biological processes as a catalyst in the active sites of metalloenzymes, as a metabolic regulator, as a mitogenic activator, and as an insulin-mimicking agent. Thus, vanadium chemistry with natural chelators, such as citrate, may have immediate implications on its role in a cellular milieu, and its action as a biological agent. In an effort to comprehend the aqueous chemistry of one of vanadium's oxidation states, namely, V(IV), implicated in its biological activity, reactions of VCl3 and citric acid were pursued in water and led to V(IV)−citrate complexes, the nature and properties of which depend strongly on the solution pH. Analytical, FT-IR, UV/vis, EPR, and magnetic susceptibility data supported the formulation of X4[{VO(H-1Cit)}2]·nH2O (H-1Cit = C6H4O74-; X = K+, n = 6 (1); X = Na+, n = 12 (2); X = NH4+, n = 2 (3)) (pH ≈ 8) and X3[{V2O2(H-1Cit)(Cit)}]·nH2O (X = K+, n = 7 (4)) (pH ≈ 5). C...