Sandwich-Type Phosphotungstates:  Structure, Electrochemistry, and Magnetism of the Trinickel-Substituted Polyoxoanion [Ni₃Na(H₂O)₂(PW₉O₃₄)₂]^11-

The novel nickel-substituted, dimeric phosphotungstate [Ni₃Na(H₂O)₂(PW₉O₃₄)₂]^11- (1) has been synthesized and characterized by IR spectroscopy, elemental analysis, and electrochemistry. X-ray single-crystal analysis was carried out on Na₁₁[Ni₃Na(H₂O)₂(PW₉O₃₄)₂]·21.25H₂O, which crystallizes in the triclinic system, space group P1̄, with a = 12.2467(6) Å, b = 16.6031(7) Å, c = 22.4017(12) Å, α = 73.9870(10)°, β = 87.6060(10)°, γ = 79.344(2)°, and Z = 2. The polyanion consists of two lacunary B-α-[PW₉O₃₄]^9- Keggin moieties linked via three nickel(II) centers and a sodium ion. The structure of 1 is composed of two fused Keggin fragments that represent different Baker−Figgis isomers (α- vs β-type). Electrochemical studies show that 1 exhibits a stable and reproducible voltammetric pattern, with a first wave featuring a chemically reversible four-electron/four-proton process. An investigation of the magnetic properties indicates that the three nickel centers exhibit ferromagnetic exchange interaction.