Magnetic anisotropy and exchange coupling in a family of isostructural FeIII2LnIII2 complexes

Abstract

The reaction of [Fe₃O(O₂CPh)₆(H₂O)₃](O₂CPh) with lanthanide/rare earth nitrate salts in the presence of triethanolamine (H₃tea) in acetonitrile/methanol solution yields a series of compounds with isostructural tetranuclear core motifs [Fe^III₂Ln^III₂(μ₃-OH)₂(teaH)₂(O₂CCPh)₆]·3MeCN (Ln = Ce (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12), Y (13)). In all cases the core topology is a defect-dicubane planar or “butterfly” Fe₂Ln₂ motif. Compounds 1–13 were investigated using a combination of experimental techniques and theoretical studies. Magnetic susceptibility measurements were carried out on all compounds. The magnetic coupling between the two Fe^III centres is antiferromagnetic, with J_FeFeca. −6.71(4) cm⁻¹, while the Fe–Ln couplings are much weaker, e.g. J_FeGd = 0.18(1) cm⁻¹. Compounds 6, 7, 8 and 13 were selected for Mössbauer studies in order to investigate the influence of isotropic (Gd^III), highly anisotropic non-Kramers and Kramers (Tb^III and Dy^III) and diamagnetic (Y^III) rare earth ions on the local environment of the Fe^III centres. Compounds 3, 6, 8 and 13 were also studied using X-Band EPR spectroscopy. For 13, with the diamagnetic Y^III ion, this made it possible to obtain the D, E, J_FeFe and g parameters for the iron centres. It is shown that the low-temperature spectra of compounds 3, 6 and 8 are determined by magnetic properties of rare-earth ions and the dipole–dipole interactions between the Ln^III ions. The Fe–Ln interactions were confirmed as very weak and dipolar in nature by the temperature dependence of EPR spectra at T > 20 K.