Polyoxoanion with octahedral germanium(IV) hetero atom: Synthesis, structure, magnetism, EPR, electrochemistry and XPS studies on the mixed-valence 14-vanadogermanate [(GeV12V2O40)-V-V-O-IV](8-)

Abstract

The mixed-valence 14-vanadogermanate [(GeV12V2O40)-V-V-O-IV](8-) (1) has been synthesized and characterized in solution by V-51-NMR, UV-vis and electrochemistry and in the solid state by IR, magnetism, EPR, XPS and elemental analysis. Single-crystal X-ray analysis was carried out on K2Na6[(GeV12V2O40)-V-V-O-IV]center dot 10H(2)O (KNa-1), which crystallizes in the orthorhombic system, space group Minim, with a = 10.9623(3) angstrom, b = 11.6205(3)angstrom, c = 20.2658(5)angstrom, and Z = 2. Polyanion 1 is composed of a central (GeO6)-O-IV octahedron which is surrounded by a total of 14 VO6 octahedra. Vanadium-51 NMR in solution results in three peaks with intensity ratio of 8:4:2 which is in complete agreement with the solid state structure. The presence of two V-IV centers was established by UV-vis, electrochemistry, magnetism, EPR, XPS and elemental analysis. Electrochemistry revealed that the two V-IV-centers in 1 are oxidized through a single well-defined step, which does not split with changes in scan rate or pH. Polyanion 1 is also an active two-electron oxidation catalyst for the coenzyme NADH at pH 8, unprecedented in polyoxometalate chemistry. Magnetic susceptibility, rnagnetization and EPR data on KNa-1 complement the X-ray and electrochemistry results by confirming the presence of two unpaired electrons per molecule of 1. The two V-IV ions possessing the spin are very weakly coupled, essentially acting as two well-isolated S = 1/2 ions. The observed g-value of 1.977 from EPR and magnetic susceptibility measurements is consistent with literature reported value for a V-IV ion, suggesting a possible ground state of 3d(x2-y2). XPS measurements on KNa-1 also confirmed the coexistence of V-V and V-IV in 1.