Magnetostructural Correlations in Tetrairon(III) Single-Molecule Magnets

We report three novel tetrairon(III) single-molecule magnets with formula [Fe-4(L)(2)(dpm)(6)] (Hdpm = 2,2,6,6-tetramethylheptane-3,5-dione), prepared by using pentaerythritol monoether ligands H3L = R'OCH2C(CH2OH)(3) with R'=allyl (1), (R,S)-2-methyl-1-butyl (2), and (S)-2-methyl-1-butyl (3), along with a new crystal phase of the complex containing H3L = 11-(acetylthio)-2,2-bis(hydroxymethyl)-undecan-1-ol (4). High-frequency EPR (HF-EPR) spectra at low temperature were collected on powder samples in order to determine the zero-field splitting (zfs) parameters in the ground S = 5 spin state. In 1-4 and in other eight isostructural compounds previously reported, a remarkable correlation is found between the axial zfs parameter D and the pitch gamma of the propeller-like structure. The relationship is directly demonstrated by I., which features both structurally and magnetically inequivalent molecules in the crystal. The dynamics of magnetization has been investigated by ac susceptometry, and the results analyzed by master-matrix calculations. The large rhombicities of 2 and 3 were found to be responsible for the fast magnetic relaxation observed in the two compounds. However, complex 3 shows an additional faster relaxation mechanism which is unaccounted for by the set of spin Hamiltonian parameters determined by HF-EPR.