Synthesis, magnetic and spectroscopic characterization of a new Fe ₇ cluster with a six-pointed star topology

A heptanuclear Fe^III₇ cluster is reported, prepared from the reaction of [Fe₃O(O₂CPh)₆(H ₂O)₃]⁺ and a combination of aliphatic aminoalcohol ligands, namely apH (3-amino-1-propanol) and hmpipH (2-hydroxymethyl piperidine). The (apH₂)[Fe₇O ₃(OH)₃(hmpip)₆(O₂CPh)₇] complex possesses a metal core resembling a metal-centered non-planar six-pointed star. Thermogravimetric analysis was used to monitor the thermal stability of 1, and shows stepwise loss of mass for this complex upon heating, while Mössbauer spectroscopy was used to evaluate the chemical environment of the Fe ions. The latter confirmed the existence of only high-spin Fe ^III ions in the cluster, as also suggested by X-ray crystallography. Dc and ac magnetic susceptibilities were measured on microcrystalline samples of 1, and the ground state for this cluster was found to be S = 21/2, with low lying excited states. Attempts to rationalize the observed spin using two different magnetostructural correlation relationships, previously derived for oxide-bridged Fe^III dimers, suggested the presence of spin frustrated pathways along the peripheral hexagon, which in turn produce a high-spin ground state. The observed S = 21/2 was finally rationalized using the results of this magnetostructural analysis, and assuming a combination of S_i = 5/2 and intermediate S_i = 3/2 spin projections. This Fe₇ cluster is the third reported example of a heptanuclear Fe cluster with this topology, featuring a tetrahedral Fe ion at the center, and joins a small family of Fe clusters based on the aliphatic ligands apH and hmpipH.