Tuning Anisotropy Barriers in a Family of Tetrairon(III) Single-Molecule Magnets with an S = 5 Ground State

摘要

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),[Fe4(L~3)_2(dpm)6] centre dot Et_2O (3 centre dot Et_2O),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(CH_2OH)_3 (H_3L~1,R = Me;H_3L~2,R = CH_2Br;H_3L~3,R = Ph;H_3L~4,R = ~tBu;Hdpm = dipivaloylmethane).The iron(III) 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 cm~(-1)) and amounts to -0.445 cm~(-1) in 1,-0.432 cm~(-1) in 2,-0.42 cm~(-1) in 3 centre dot Et_2O,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 centre dot Et_2O,5.95 K in 4,and 3.5 K in 5.Both |D| and U_(eff) are found to increase with increasing helical pitch of the Fe(O_2Fe)_3 core.The fourth-order longitudinal anisotropy parameter B_4~O,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.