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

Abstract

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 = C6H4O7 4-

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). Complex 2 crystallizes in space group P21/c, a = 11.3335(9) Å, b = 15.788(1) Å, c = 8.6960(6) Å, β = 104.874(3)°, V = 1503.8(2), Z = 2. Complex 3 crystallizes in space group P1̄, a = 9.405(1) Å, b = 10.007(1) Å, c = 13.983(2) Å, α = 76.358(4)°, β = 84.056(4)°, γ = 66.102(4)°, V = 1169.2(3), Z = 2. Complex 4 crystallizes in space group P21nb, a = 9.679(4) Å, b = 19.618(8) Å, c = 28,30(1) Å, V = 5374.0(4), Z = 8. The X-ray structures of 1-4 are V2O2 dimers, with the citrate displaying varying coordination numbers and modes. 1 exhibits a small ferromagnetic interaction, whereas 4 exhibits an antiferromagnetic interaction between the V(IV) ions. 1 and 4 interconvert with pH, thus rendering the pH a determining factor promoting variable structural, electronic, and magnetic properties in V(IV) - citrate species. The observed aqueous behavior of 1-4 is consistent with past solution speciation studies, and contributes to the understanding of significant aspects of the biologically relevant vanadium(IV) - citrate chemistry.