Single-ion and molecular contributions to the zero-field splitting in an iron(III)-oxo dimer studied by single crystal W-band EPR.

Detailed knowledge of the type and strength of pair interactions between high-spin metal ions is paramount to the understanding and design of molecular magnetic materials. In this work, the anisotropic magnetic interactions in a beta-diketonate-alkoxide iron(III) dimer compound, [Fe2(OCH3)2(dbm)4, Hdbm=dibenzoylmethane] (Fe2) have been investigated by single crystal electron paramagnetic resonance (EPR) in the W-band (at 95GHz). The diamagnetic substitution method was employed using the isomorphous gallium(III)-based compound doped with iron(III) to produce Ga-Fe dimers (GaFe). The single-ion zero-field splitting (ZFS) tensor could be separately determined in GaFe with the iron ion in a local environment quasi-identical to the one in Fe2. Its principal directions are found to point in arbitrary directions, uncorrelated with the Fe-O bonds. The Fe2 EPR spectra consist of transitions within the lowest multiplet states S=1,2,3, which were analyzed using the full spin Hamiltonian description of an exchange coupled pair of s=5/2 spins. The anisotropic spin-spin interaction tensor of Fe2 possesses a principal axis close to the Fe-Fe direction and was shown to arise both from through-space (dipolar) and through-bond (anisotropic exchange) contributions. The latter involves an rhombic component JE=(JX-JY)/2 approximately 0.093 cm-1 of magnitude comparable to the dipolar interaction, and even to the rhombic part of the single-ion ZFS (E=0.097 cm-1). Our results show that the anisotropic exchange, usually neglected for S-type ions, is significant for the anisotropic interactions in exchange-coupled iron(III) clusters, including the Fe4 and Fe8 families of single-molecule magnets and the antiferromagnetic iron wheels.