An ‘Empty’ [Fe 16 ] Cage and its Transformation into [Fe 17 ] and [Fe 30 ]

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, [Fe 8 ], [Fe 17 ], [Fe 30 ] and [Fe 34 ], have been isolated from pyridine or analogous heterocyclic amine solvents. Here, we report the isolation of an [Fe 16 ] cluster from dmf. The complex is ‘isostructural’ with the previously reported [Fe 17 ] molecular metal oxide, but with the central, tetrahedral Fe III ion absent. Antiferromagnetic exchange interactions between the tetrahedral and octahedral Fe III ions in [Fe 16 ] 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 [Fe 17 ] containing an additional (antiferromagnetically coupled) s = 5/2 ion. Dissolution of [Fe 16 ] in pyridine regenerates [Fe 17 ] or the larger [Fe 30 ], depending on reaction conditions, demonstrating for the first time that these species can be interconverted.