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 s tudies of vanadium(III) with a biologically relevant,. organic, dicarboxyli c 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(O-2)(C4H3O5)](2). nH(2)O ( Cat K+, n = 4, 1; Cat = NH4+, n = 3, 2) and K-2[VO(O-2)(C4H4O5)](2). 2H(2)O (3). All three complexes were:characterized by elemental analysis, FT-IR, and UV/visible spectroscopies. Compound 1 crystallizes in the monoclinic sp ace group P2(1)/c, with a 8.380(5) Angstrom, b = 9.252(5) Angstrom, c = 13. 714(8) Angstrom, beta = 93.60(2)degrees, V = 1061(1) Angstrom (3), and Z = 4. Compound 2 crystallizes in the triclinic space group Pi, with a = 9.158( 4) Angstrom (3), b = 9.664(4), c = 14.185(6) Angstrom, alpha = 104.81(1)deg rees, beta = 90.31(1)degrees; gamma = 115.643(13)degrees, V = 1085.0(7) Ang strom (3), and Z = 2. Compound 3 crystallizes in the monoclinic space group P2(1)/c, with a 9.123(8) Angstrom, b = 9.439(8) Angstrom, c = 10.640(9) An gstrom, beta = 104.58(3)degrees, V = 887(1) Angstrom (3), and Z = 2. The X- ray structures showed that, in 1 and 2; the dimers consist of two (V-V=O)(2 )O-2 rhombic units to which two malate ligands are attached. The ligands ar e triply deprotonated and, as such, they coordinate,to vanadium(V), promoti ng a pentagonal bipyramidal geometry. In 3, the dimeric (V-V=O)(2)O-2 rhomb ic unit persists, with the two doubly deprotonated malate ligands coordinat ed to the vanadium(V) ions. UV/vis and EPR spectroscopic studies on the int ermediate blue solutions of the synthesis reactions of 1-3 support the exis tence of vanadyl-containing dimeric species. These species further react wi th H2O2 to yield oxidation of (V2O2)-O-IV to (V2O2)-O-V and coordination of the peroxide to vanadium(V). From the collective data on 1-3, it appears t hat pH acts as a decisive factor in dictating;the structural features of th e isolated complexes. The details of the introduced structural differentiat ion in the reported complexes, and their potential relevance to vanadium(V) dicarboxylate systems in biological media are dwelled-on.