Pseudo-tetrahedral vs. pseudo-octahedral ErIII single molecule magnets and the disruptive role of coordinated TEMPO radical

Abstract

Er^III complexes are potential candidates for high-performance single molecule magnets (SMMs) just after Dy^III. Herein, we thoroughly explore the under-represented class of neutral pseudo-tetrahedral Er^III SMMs and demonstrate their exceptional slow magnetization dynamics. We found that this dynamics is solely controlled by the Raman-like relaxation mechanism in the investigated temperature and frequency range which is an uncommon observation in the field of Ln-based SMMs. We also compare the molecular magnetic memory effect in the form of a waist-restricted magnetic hysteresis loop of the reported pseudo-tetrahedral Er^III SMMs with an example of a hexa-coordinate pseudo-octahedral one, that does not show this effect. Overall, we report four new neutral mononuclear Er^III complexes – three tetra-coordinate: [Er^III(TTBP)₃(THF)] (1) (TTBP⁻ = 2,4,6-tri-tert-butyl-phenolate), [Er^III(TTBP)₃(TEMPO)] (2) (TEMPO = 2,2,6,6-tetramethylpiperidine 1-oxyl) and [Er^III(BHT)₃(THF)] (3) (BHT⁻ = 2,6-di-tert-butyl-4-methylphenolate) and one hexa-coordinate: [Li(THF)₂]₂[Er^III(N₃N)Cl₂] complex (4) (N₃N = tris(N-trimethylsilyl-2-amidoethyl)amine; [(Me₃SiNCH₂CH₂)₃N]³⁻). Finally, by comparing [Er^III(TTBP)₃(THF)] (1) and its radical-functionalized congener [Er^III(TTBP)₃(TEMPO)] (2), we show that adding an additional spin on the ligand of an Er^III SMM completely disrupts the slow magnetization dynamics of the parent compound.