Magnetostructural correlations in a family of tetranuclear iron(III) SMMs

The ability to control the anisotropy of Single-Molecule Magnet is one of the key issue in the field of molecular magnetism as it might lead to systems with higher magnetization reversal barrier and thus to higher blocking temperature. We present here multifrequency (95 GHz - 345 GHz) variable temperature (5-30 K) HF-EPR spectra on a family of tetranuclear iron clusters characterised by S=5 ground state and Single Molecule Magnet behavior at low temperature. Single crystal W-Band spectra on Fe4(OMe)6(dpm)6 - where Hdpm = dipivaloylmethane - showed the presence of more than one isomers with slightly differing anisotropies and confirmed the supposition about the orientation of the easy axis perpendicular to the molecular plane. HF-EPR spectra of clusters of general formula Fe4(L)2(dpm)6 (H3L = MeC(CH2OH)3, PhC(CH2OH)3, BrCH2C(CH2OH)3), obtained by site-specific replacement of the six methoxide bridges in Fe4(OMe)6(dpm)6, clearly demonstrates that the ligand substitution results in an increased anisotropy of the S = 5 ground spin state (D ~ -0.4 cm -1 vs. - 0.2 cm -1 ). These results are discussed in conjunction with both DC and AC magnetic data which show the onset of slow relaxation and the observation of pure quantum tunneling of the magnetization at higher temperature compared to the original derivative, thus essentially confirming the picture coming out from HF-EPR spectra analysis. The analysis of these results on the basis of the molecular structure of the different complexes allowed us to derive multiple magnetostructural correlations involving both 2nd- (D) and 4th-order (B4 0 ) anisotropy parameters. It turned out that while the D parameter is mainly determined by the local anisotropy components along the threefold molecular axis (in accordance with standard projection formulae), B4 0 displays a more complex dependence on the single ion ZFS tensors. We show that the observed variation of D and B4 0 can be accurately reproduced by simply varying the orientation of the peripheral Di tensors along the Fe-Fe direction. In the Fe4 family this reorientation is presumably triggered by the gradual increase of helical "pitch" observed along the series. These results have important consequence for the targeted design of molecules showing Single Molecule Magnet behavior at higher temperatures.