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

Abstract

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 β-diketonate-alkoxide iron(III) dimer compound, [Fe 2(OCH 3) 2(dbm) 4, Hdbm = dibenzoylmethane] (Fe2) have been investigated by single crystal electron paramagnetic resonance (EPR) in the W-band (at 95 GHz). 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 J E = ( J X − J Y )/2 ≈ 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 Fe 4 and Fe 8 families of single-molecule magnets and the antiferromagnetic iron wheels.