pH-Specific Synthesis of a Dinuclear Vanadium(V)−Peroxo−Citrate Complex in Aqueous Solutions: pH-Dependent Linkage, Spectroscopic and Structural Correlations with Other Aqueous Vanadium(V)−Peroxo−Citrate and Non-Peroxo Species

Aqueous reactions of V2O5 or VCl3 in the presence of the physiological citric acid and hydrogen peroxide, in a pH
specific fashion, afforded a new vanadium(V)−peroxo−citrate material isolated in a pure crystalline form. Elemental
analysis pointed to the molecular formulation (NH4)6[VV2O2(O2)2(C6H4O7)2]·4.5H2O (1). Complex 1 was further
characterized by UV−vis, FT-IR, and X-ray crystallography. Compound 1 crystallizes in the monoclinic space group
C2/c with a = 12.391(5) Å, b = 15.737(7) Å, c = 17.102(7) Å, β = 110.84(1)°, V = 3117(1) Å3, and Z = 4. The
structure of the anionic assembly consists of a planar VV2O2 core with two fully deprotonated citrates bound to it
through the central carboxylate and alkoxide moieties as well as one of the terminal carboxylate groups. The
presence of one peroxide group attached to each vanadium(V) renders the geometry around each metal center
pentagonal bipyramidal. Key structural and spectroscopic features of 1 correlate with those seen in the peroxo
congener and low-pH analogue (NH4)2[VV2O2(O2)2(C6H6O7)2]·2H2O (3), in which all terminal carboxylate groups are
protonated. In solution, simple pH-dependent transformation of 1 to 3 attests to their participation in the requisite
speciation and potentiates the presence of other similar peroxo analogues not yet isolated and characterized. The
reactivity of 1 through transformation reactions, yielding a plethora of well-characterized species, establishes a
linkage among various species with the same or different vanadium oxidation states. Collectively, the data reflect
soluble forms of vanadium with peroxide and citrate that contribute to the requisite pH-dependent distribution of
that metal ion and likely influence biological processes.