A Novel Banana-Shaped Mixed-Metal Co/Fe Polyoxometalate Cluster.

The synthesis and characterization of a Co/Fe mixed-metal banana-shaped polyoxometalate with the formula [(Co_2.5Fe_0.5(H₂O)PW₉O₃₄)₂(PW₆O₂₆)]^16- (Co₅Fe) is reported. This transition-metal-substituted polyoxometalate readily assembles from its components in a one-pot reaction and crystallizes in the monoclinic space group P2₁/c. The structure of Co₅Fe can be considered a double sandwich composed by two B-α-{Co_2.5Fe_0.5PW₉O₄₀} Keggin units, in which one coordinatively saturated octahedral metal position is equally occupied by Co(II) and Fe(III) ions with a 50 % of site occupancy. These Keggin units are linked via a hexalacunary Keggin unit {PW₆O₂₆}. Single crystal X-ray diffraction and magnetic measurements support the proposed atom arrangement within the crystal structure. Magnetic measurements of these double trimeric unit {Co_2.5Fe_0.5O₁₃}₂ show a combination of antiferromagnetic interactions, the presence of spin frustration, and the first-order spin-orbit coupling of Co(II) ions. Electrocatalytic water oxidation measurements show that Co₅Fe displays low stability in both homogeneous and heterogeneous conditions. This is evidenced by the constant increase on the catalytic currents over time together with the appearance of polyoxometalate-derived electrode-bound species that can be responsible for the observed catalytic activity.