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

Polymetallic clusters of FeIII 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 FeIII ion absent. Antiferromagnetic exchange interactions between the tetrahedral and octahedral FeIII 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.