Magnetic anisotropy and exchange coupling in a family of isostructural Fe~(III) _2Ln~(III) _2 complexes

摘要

The reaction of [Fe_3O(O_2CPh)_6(H 2O)_3](O_2CPh) with lanthanide/rare earth nitrate salts in the presence of triethanolamine (H_3tea) in acetonitrile/methanol solution yields a series of compounds with isostructural tetranuclear core motifs [Fe~(III) _2Ln~(III) _2(μ_3-OH)_2(teaH)_2(O _2CCPh)_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" Fe_2Ln_2 motif. Compounds 1~(-1)3 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_(FeFe)ca. -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 M?ssbauer 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, JFeFe 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.