A New Dinuclear Vanadium(V)−Citrate Complex from Aqueous Solutions. Synthetic, Structural, Spectroscopic, and pH-Dependent Studies in Relevance to Aqueous Vanadium(V)−Citrate Speciation

Vanadium interactions with low molecular mass binders in biological fluids entail the existence of vanadium species with variable chemical and biological properties. In the course of efforts to elucidate the chemistry related to such interactions, we have explored the oxidative chemistry of vanadium(III) with the physiologically relevant tricarboxylic citric acid. Aqueous reactions involving VCl3 and anhydrous citric acid, at pH ∼ 7, resulted in blue solutions. Investigation into the nature of the species arising in those solutions revealed, through UV/visible and EPR spectroscopies, oxidation of vanadium(III) to vanadium(IV). Further addition of H2O2 resulted in the oxidation of vanadium(IV) to vanadium(V), and the isolation of a new vanadium(V)−citrate complex in the form of its potassium salt. Analogous reactions with K4[V2O2(C6H4O7)2]·6H2O and H2O2 or V2O5 and citrate at pH ∼ 5.5 afforded the same material. Elemental analysis pointed to the molecular formulation K4[V2O4(C6H5O7)2]·5.6H2O (1). Complex 1 was further characterized by FT-IR and X-ray crystallography. 1 crystallizes in the triclinic space group P1̄, with a = 11.093(4) Å, b = 9.186(3) Å, c = 15.503(5) Å, α = 78.60(1)°, β = 86.16(1)°, γ = 69.87(1)°, V = 1454.0(8) Å3, and Z = 2. The X-ray structure of 1 reveals the presence of a dinuclear vanadium(V)−citrate complex containing a VV2O2 core. The citrate ligands are triply deprotonated, and as such they bind to vanadium(V) ions, thus generating a distorted trigonal bipyramidal geometry. Binding occurs through the central alkoxide and carboxylate groups, with the remaining two terminal carboxylates being uncoordinated. One of those carboxylates is protonated and contributes to hydrogen bond formation with the deprotonated terminal carboxylate of an adjacent molecule. Therefore, an extended network of hydrogen-bonded VV2O2-core-containing dimers is created in the lattice of 1. pH-dependent transformations of 1 in aqueous media suggest its involvement in a web of vanadium(V)−citrate dinuclear species, consistent with past solution speciation studies investigating biologically relevant forms of vanadium.