Mapping the energy spectrum of the spin states of mixed-valent (Fe8)n- via pulsed field magnetization

The electronic structure of a family of octanuclear FeIII-complexes of the general formula (Fe8(μ4-O)4(μ-r-R-pz)12X4) ((Fe8)0) and its redox-modified, mixed-valence (Fe8)n- derivatives, where R = H, Me, Et, F, CI, Sr, I, etc. and X = F, CI, Sr, NCS, NCO, N3, has recently been modeled by a an effective Hamiltonian consisting of two dominant exchange interactions (1). The ground state properties (from Stot = 0 to 7) and magnetic energy level spacing of the Hamiltonian, and hence predicted magnetic properties, are widely tunable via choice of J's. The corresponding (Fe8)n- anionic complexes with n = 1 - 4 are accessible electrochemically, allowing their in situ spectroelectrochemical characterization. The singly-reduced anions (Fe8)1- of the R = H, Cl and X = Cl species have also been prepared chemically via reduction with a stoichiometric amount of (BH4)-, and characterized crystallographically; the structure of the Fe8-cluster remains unaffected by the reduction, with most bond lengths differences within experimental error. Their Moessbauer spectroscopic analysis has pointed to the reduction taking place primarily within the Fe4O4-cubane, with charges delocalized over the four Fec sites in the Moessbauer timescale. In contrast, the (Fe8)1- and (Fe8)2- species with R = Cl and X = NCS show a reduction at the outer, Feo-sites, generating one or two localized Feo-centers.