Concepedia

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

S. Accorsi, Anne‐Laure Barra, Andréa Caneschi, Guillaume Chastanet, Andrea Cornia, Antonio C. Fabretti, Dante Gatteschi, Cecilia Mortalò, Emiliano Olivieri, Francesca Parenti,

Journal of the American Chemical Society · 2006 · 208 citations · 62 references

DOI Full text

Open access

TL;DR

Tetrairon(III) single‑molecule magnets with a propeller‑like structure display tunable magnetic anisotropy barriers in both height and shape. The authors synthesized four Fe₄ clusters by reacting a common precursor with tripodal ligands, producing centered‑triangular, alkoxo‑bridged assemblies that propagate antiferromagnetic interactions to yield an S = 5 ground state, and used spin‑Hamiltonian calculations to link changes in helical pitch to modulation of single‑ion anisotropies. High‑frequency EPR and AC susceptibility measurements reveal that the axial zero‑field splitting parameter D is negative and varies from –0.445 cm⁻¹ to –0.27 cm⁻¹, while the effective anisotropy barriers Ueff/kB range from 17.0 K to 3.5 K, increasing with helical pitch, and the fourth‑order parameter B4(0) shifts from positive in cluster 1 to negative in cluster 5, with cluster 4 crystals containing at least two geometric isomers.

Abstract

Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, 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.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 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(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(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.