pH-Dependent Investigations of Vanadium(V)−Peroxo−Malate Complexes from Aqueous Solutions. In Search of Biologically Relevant Vanadium(V)−Peroxo Species

The established biochemical potential of vanadium has spurred considerable research interest in our lab, with specific focus on pertinent synthetic studies of vanadium(III) with a biologically relevant, organic, dicarboxylic acid, malic acid, in aqueous solutions. Simple reactions between VCl₃ and malic acid in water, at different pH values, in the presence of H₂O₂, led to the crystalline dimeric complexes (Cat)₄[VO(O₂)(C₄H₃O₅)]₂·nH₂O (Cat = K⁺, n = 4, 1; Cat = NH₄⁺, n = 3, 2) and K₂[VO(O₂)(C₄H₄O₅)]₂·2H₂O (3). All three complexes were characterized by elemental analysis, FT-IR, and UV/visible spectroscopies. Compound 1 crystallizes in the monoclinic space group P2₁/c, with a = 8.380(5) Å, b = 9.252(5) Å, c = 13.714(8) Å, β = 93.60(2)°, V = 1061(1) ų, and Z = 4. Compound 2 crystallizes in the triclinic space group P1̄, with a = 9.158(4) Å, b = 9.669(4) Å, c = 14.185(6) Å, α = 104.81(1)°, β = 90.31(1)°, γ = 115.643(13)°, V = 1085.0(7) ų, and Z = 2. Compound 3 crystallizes in the monoclinic space group P2₁/c, with a = 9.123(8) Å, b = 9.439(8) Å, c = 10.640(9) Å, β = 104.58(3)°, V = 887(1) ų, and Z = 2. The X-ray structures showed that, in 1 and 2, the dimers consist of two (V^VO)₂O₂ rhombic units to which two malate ligands are attached. The ligands are triply deprotonated and, as such, they coordinate to vanadium(V), promoting a pentagonal bipyramidal geometry. In 3, the dimeric (V^VO)₂O₂ rhombic unit persists, with the two doubly deprotonated malate ligands coordinated to the vanadium(V) ions. UV/vis and EPR spectroscopic studies on the intermediate blue solutions of the synthesis reactions of 1−3 support the existence of vanadyl-containing dimeric species. These species further react with H₂O₂ to yield oxidation of V^IV₂O₂ to V^V₂O₂ and coordination of the peroxide to vanadium(V). From the collective data on 1−3, it appears that pH acts as a decisive factor in dictating the structural features of the isolated complexes. The details of the introduced structural differentiation in the reported complexes, and their potential relevance to vanadium(V) dicarboxylate systems in biological media are dwelled on.