Polyoxoanion with Octahedral Germanium(IV) Hetero Atom: Synthesis, Structure, Magnetism, EPR, Electrochemistry and XPS Studies on the Mixed-Valence 14-Vanadogermanate [GeVV 12VIV 2O40]8−

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The mixed-valence 14-vanadogermanate [GeVV 12VIV 2O40]8− (1) has been synthesized and characterized in solution by 51V-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[GeVV 12VIV 2O40]·10H2O (KNa-1), which crystallizes in the orthorhombic system, space group Immm, with a=10.9623(3) A, b=11.6205(3) A, c=20.2658(5) A, and Z=2. Polyanion 1 is composed of a central GeIVO6 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 VIV centers was established by UV–vis, electrochemistry, magnetism, EPR, XPS and elemental analysis. Electrochemistry revealed that the two VIV-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, magnetization 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 VIV 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 VIV ion, suggesting a possible ground state of $$3d_{x^{2}-y^{2}}.$$ XPS measurements on KNa-1 also confirmed the coexistence of VV and VIV in 1.