An 'Empty' [Fe₁₆] Cage and its Transformation into [Fe₁₇] and [Fe₃₀]

Polymetallic clusters of Fe^III constructed without organic bridging ligands are an emerging class of molecules with structures akin to iron oxide minerals, possessing large spin ground states and/or displaying spin frustration effects. All previous examples, [Fe₈], [Fe₁₇], [Fe₃₀] and [Fe₃₄], have been isolated from pyridine or analogous heterocyclic amine solvents. Here, we report the isolation of an [Fe₁₆] cluster from dmf. The complex is 'isostructural' with the previously reported [Fe₁₇] molecular metal oxide, but with the central, tetrahedral Fe^III ion absent. Antiferromagnetic exchange interactions between the tetrahedral and octahedral Fe^III ions in [Fe₁₆] lead to a spin ground state of S = 20, the highest yet for this family of materials, consistent with the S = 35/2 found for [Fe₁₇] containing an additional (antiferromagnetically coupled) s = 5/2 ion. Dissolution of [Fe₁₆] in pyridine regenerates [Fe₁₇] or the larger [Fe₃₀], depending on reaction conditions, demonstrating for the first time that these species can be interconverted.