A Three-Pronged Attack To Investigate the Electronic Structure of a Family of Ferromagnetic Fe4Ln2 Cyclic Coordination Clusters: A Combined Magnetic Susceptibility, High-Field/High-Frequency Electron Paramagnetic Resonance, and 57Fe Mossbauer Study

We present the synthesis, structure, magnetic properties, as well as the Mossbauer and electron paramagnetic resonance studies of a ring-shaped [(Fe4L2III)-L-III-(Htea)(4)(mu-N-3)(4)(N-3)(3)(piv)(3)] (Ln = Y 1, Gd 2, Tb 3, Dy 4, Ho 5, Er, 6) coordination cluster. The Dy, Tb, and Ho analogues show blocking of the magnetization at low temperatures without applied fields. The anisotropy of the 3d ion and the exchange interaction between 3d and 4f ions in Fe4Ln2 complexes are unambiguously determined by high-field/high-frequency electron paramagnetic resonance measurements at low temperature. Ferromagnetic exchange interaction J(Fe-Ln) is found which decreases upon variation of the Ln ions to larger atomic numbers. This dependence is similar to the behavior shown in the effective barrier values of complexes 3-5. Further information about the anisotropy of the Ln(3+) ions was gathered with Fe-57 Mossbauer spectroscopy, and the combination of these methods provides detailed information regarding the electronic structure of these complexes.