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 i on chelator. Vanadium participates in biological processes as a catalyst in the active sites of metalloenzymes, as a metabolic regulator, as a mitogen ic activator, and as an insulin-mimicking agent. Thus, vanadium chemistry w ith 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 s tates, namely, V(IV), implicated in its biological activity, reactions Of V Cl3 and citric acid were pursued in water and led to V(IV)-citrate complexe s, the nature and properties of which depend strongly on the solution pH. A nalytical, FT-IR, UV/vis, EPR, and magnetic susceptibility data supported t he formulation of X-4[{VO(H-(1)Cit)}(2)]. nH(2)O (H(-1)Cit = C(6)H4O(7)(4-) ; X = K+, n = 6 (1); X = Na+, n = 12 (2); X = NH4+, n = 2 (3)) (pH approxim ate to 8) and X-3[{V2O2(H(-1)Cit)(Cit)}]. nH(2)O (X = K+, n = 7 (4)) (pH ap proximate to 5). Complex 2 crystallizes in space group P2(1)/c, a = 11.3335 (9) , b = 15.788(1) Angstrom, c = 8.6960(6) Angstrom, beta = 104.874(3)degr ees, V = 1503.8(2), Z = 2. Complex 3 crystallizes in space group P (1) over bar, a = 9.405(1) Angstrom, b = 10.007(1) Angstrom, c = 13.983(2 )Angstrom , alpha = 76.358(4)degrees, beta = 84.056(4)degrees, gamma = 66.102(4)degre es, V = 1169.2(3), Z = 2. Complex 4 crystallizes in space group P2(1)nb, a = 9.679(4) Angstrom, b = 19.618(8) Angstrom, c = 28.30(1) Angstrom, V = 537 4.0(4), Z = 8. The X-ray structures of 1-4 are V2O2 dimers, with the citrat e displaying varying coordination numbers and modes. 1 exhibits a small fer romagnetic interaction, whereas 4 exhibits an antiferromagnetic interaction between the V(IV) ions. 1 and 4 interconvert with pH, thus rendering the p H a determining factor promoting variable structural, electronic, and magne tic 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 vanad ium(IV)-citrate chemistry.