Single-molecule Magnet Behavior of a Tetranuclear Iron(III) Complex. The Origin of Slow Magnetic Relaxation in Iron(III) Clusters

The synthesis, crystal structure, and magnetic characterization of a novel tetranuclear Fe(III) methoxo-bridged cluster Fe4(OMe)6(dpm)6 (Hdpm = dipivaloylmethane) is reported. The cluster has a ground spin state of S = 5, which is selectively populated below 20 K. High-field EPR spectra revealed that the system has a uniaxial magnetic anisotropy, corresponding to a zero field splitting parameter D = -0.2 cm-1 of the S = 5. Such anisotropy below 1 K gives rise to the slow relaxation of the magnetization similar to that of super-paramagnets. To study the origin of the magnetic anisotropy the projection of the single-ion and dipolar contributions has been evaluated to the zfs of the ground state. The zfs tensors of the 3 structurally independent Fe(III) centers have been calcd. from the coordination geometry and spectroscopic data using the angular overlap model. To test the reliability of the approach high-field EPR spectra of the parent monomer Fe(dpm)3 have been recorded to compare the calcd. and exptl. zfs parameters.