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 [(Co2.5Fe0.5(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‐{Co2.5Fe0.5PW9O40} 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 {Co2.5Fe0.5O13}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. A novel synthetic approach resulted in the formation of a new banana‐shaped mixed‐metal CoII/FeIII polyoxometalate Na16[(Co2.5Fe0.5(H2O)PW9O34)2(PW6O26)] ⋅ 40H2O through a one‐pot reaction. Single crystal analysis and physical characterization suggest that the CoII/FeIII‐oxo core is formulated as {Co2.5Fe0.5O13}, whereas magnetic measurements indicate dominant antiferromagnetic interaction, spin frustration together with spin‐orbit coupling of Co(II). Finally, the polyoxometalate displays low stability under harsh OER conditions.