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BIOLOGY, CHEMISTRY AND SOFT MATTER 2009Magnetostructural correlations in Tetrairon(III) single-molecule magnetsSlow magnetic relaxation in molecular systems is a livelyresearch area which spans the interface between chemistry,physics and material science. This phenomenon is observedin two main families of compounds: single-molecule magnets(SMM) and single-chain magnets (SCM). The persistenceof magnetization in such systems is limited to lowtemperature (below about 4.2 K), but can be in principleexploited for applications in the field of magnetic storageand information processing. Tetrairon(III) complexes withformula [Fe 4 (L) 2 (dpm) 6 ],are providing a growing class ofSingle Molecule Magnets displaying unprecedented syntheticflexibility and ease of functionalization ( Hdpm =2,2,6,6 − tetramethyl-heptane-3,5-dione).Here we report on three novel derivatives prepared byusing as bridging ligands pentaerythritol monoethers,H 3 L = R’-O-CH 2 C(CH 2 OH) 3 with R’=allyl (1), (R,S))-2-methyl-1-butyl (2), and S-2-methyl-1-butyl (3) along witha new polymorph the complex containing 11-(acetylthio)-2,2-bis(hydroxymethyl) undecan-1-ol ligands (4b) [Gregoliet al., Chem. Eur. J. 15, 6456 (2009)]. High-FrequencyEPR (HF-EPR) spectra have been collected, at two frequencies(190 and 230 GHz) and several temperatures between 5and 30 K, on polycrystalline samples of the four complexesin order to determine the zero-field splitting (zfs) parametersin the ground spin state. The spectra obtained show thetypical behavior of systems with an S = 5 ground spin statecharacterized by an easy-axis type anisotropy (D < 0):H = µ 0 B.g.S + DS 2 z + E(S 2 x − S 2 y) + B 0 4 O0 4 (17)where O 0 4 is a Stevens operator, while D, E and B0 4 arethe crystal field parameters defining the anisotropy of thesystem. Setting an isotropic Landé factor g = 2.0, the bestsimulations of experimental spectra were obtained with thefollowing parameters (in cm −1 ): D = −0.417, E = 0.015,B 0 4 = +1.3 × 10−5 for (1a), D = −0.435, E = 0.009,B 0 4 = +0.9 × 10−5 for (1b), D = −0.449, E = 0.030,B 0 4 = +2.4 × 10−5 for (2), D = −0.442, E = 0.0312,B 0 4 = +1.65 × 10−5 for (3) and D = −0.412, E = 0.006,B 0 4 = +1.8 × 10−5 for (4b)(figure 1). (1a) and (1b) standfor the two structurally inequivalent molecules present inthe cell of crystal (1). Experimental and calculated spectrafor complex (2) at 230 GHz are displayed in figure 141.In (1-3) and (4b), and in other six isostructural compoundspreviously reported, a remarkable correlation is found betweenthe axial zfs parameter D and the helical pitch γof the propeller-like structure (figure 142). γ is definedas the average dihedral angle between the mean Fe4 planeand the three FeO 2 Fe ”blades”.The relationship is directlydemonstrated by (1), which features both structurally andmagnetically inequivalent molecules in the crystal. Thetwo polymorphs of (4) ((4a,b)) span the whole range ofanisotropies for [Fe 4 (L) 2 (dpm) 6 ] complexes, suggestingthat crystal packing effects may be largely responsible forthe observed structural and magnetic differences.The dynamics of the magnetization in the four complexeshas been investigated by AC susceptometry, and the resultsanalyzed by master-matrix calculations. The large rhombicityof (2) and (3) is found responsible for the fast magneticrelaxation observed in the two compounds. However, complex(3) shows an additional faster relaxation mechanismwhich is unaccounted for by the set of spin-Hamiltonianparameters determined by HF-EPR.Figure 141: HF-EPR experimental (bold) and simulated powderspectra recorded at 230 GHz on derivative (2).Figure 142: Axial anisotropy D versus helical pitch γ for thetwelve Tetrairon(III) propellers so far characterized. The redpoints correspond to the four derivatives discussed here. The solidline correspond to the best fit for the derivatives featuring twotripodal ligands.A.L. Barra, P. NeugebauerA. Cornia, L. Gregoli, C. Dianeli (University of Modena, Italy), R. Sessoli (University of Florence, Italy)98