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

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posted on 2001-06-19, 00:00 authored by M. Kaliva, T. Giannadaki, A. Salifoglou, C. P. Raptopoulou, A. Terzis, V. Tangoulis
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) Å3, 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) Å3, 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) Å3, and Z = 2. The X-ray structures showed that, in 1 and 2, the dimers consist of two (VV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 (VV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 13 support the existence of vanadyl-containing dimeric species. These species further react with H2O2 to yield oxidation of VIV2O2 to VV2O2 and coordination of the peroxide to vanadium(V). From the collective data on 13, 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.

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