Fe-Gd Ferromagnetic Cyclic Coordination Cluster [Fe III 4 Gd III 4 (teaH) 8 (N 3 ) 8 (H 2 O)] with Magnetic Anisotropy─Theory and Experiment.

The synthesis, structural, and magnetic characterization of [Fe III 4 Ln III 4 (teaH) 8 (N 3 ) 8 (H 2 O)] (Ln = Gd and Y) and the previously reported isostructural Dy analogue are discussed. The commonly held belief that both Fe III and Gd III can be regarded as isotropic ions is shown to be an oversimplification. This conclusion is derived from the magnetic data for the Y III analogue in terms of the zero-field splitting seen for Fe III and from the fact that the magnetic data for the new Gd III analogue can only be fit employing an additional anisotropy term for the Gd III ions. Furthermore, the Fe 4 Gd 4 ring shows slow relaxation of magnetization. Our analysis of the experimental magnetic data employs both density functional theory as well as the finite-temperature Lanczos method which finally enables us to provide an almost perfect fit of magnetocaloric properties.