A Diferrous Single‐Molecule Magnet

The diferrous complex [Fe2(acpypentO)(NCO)3] (1),(acpypentO– = the anion of 1,5‐bis[2‐pyridyl(1‐ethylimino)]pentane‐3‐ol) was studied by spectroscopic (dual‐mode X‐band EPR and 57Fe‐Mössbauer) and magnetic (AC magnetic susceptibility) techniques. Complex 1 (A. K. Boudalis et al., Inorg. Chem. 2004, 43, 1574) was previously shown to exhibit an intramolecular ferromagnetic coupling with important single‐ion anisotropies with marked differences in the single‐ion electronic parameters of the two iron sites. In this contribution, additional studies are carried out to probe its dynamic magnetic properties. Mössbauer spectra recorded at liquid helium temperatures indicate slow paramagnetic relaxation and a ground state of the system characterized by a non‐Kramers doublet with an Ising‐type anisotropy, a condition which favours single‐molecule magnet (SMM) behaviour. A frequency‐dependent out‐of‐phase signal observed below 3 K in AC susceptibility measurements is in line with such a picture. The out‐of‐phase signal, however, is relatively weak in comparison with the in‐phase signal. Dual mode X‐band EPR spectroscopy revealed characteristic signals, the analysis of which suggests inhomogeneities in the system. For some molecules, the splitting Δ0 of the non‐Kramers ground doublet is vanishingly small, which gives rise to out‐of‐phase signals in AC susceptibility measurements and magnetically split Mössbauer spectra in the absence of external magnetic fields. Other molecules, however, are characterized by a nonnegligible Δ0, which gives rise to the dual mode X‐band EPR signals. For these molecules the magnetic relaxation is fast. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)