Tuning anisotropy barriers in a family of tetrairon(III) single-molecule magnets with an S=5 ground state

Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe-4(L-1)(2)(dpm)(6)] (1), [Fe-4(L-2)(2)(dPM)(6)] (2), [Fe-4(L-3)(2)(dPM)(6)]center dot Et2O (3 center dot Et2O), and [Fe-4(OEt)(3)(L-4)(dPM)(6)] (4) have been prepared by reaction of [Fe-4(OMe)(6)(dpm)(6)] (5) with tripodal ligands R-C(CH2OH)(3) (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = 'Bu; Hdpm = dipivaloylmethane). The iron(Ill) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 kcm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3 center dot Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier U-eff determined by AC magnetic susceptibility measurements is U-eff/k(B) = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3 center dot Et2O, 5.95 K in 4, and 3.5 K in 5. Both vertical bar D vertical bar and Ueff are found to increase with increasing helical pitch of the Fe(O2Fe)(3) core. The fourth-order longitudinal anisotropy parameter B-4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.