Insight into D6h Symmetry: Targeting Strong Axiality in Stable Dysprosium(III) Hexagonal Bipyramidal Single-Ion Magnets

Abstract

Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph₃ SiO^- (Ph₃ SiO^- =anion of triphenylsilanol) and the 2,4-di-^t Bu-PhO^- (2,4-di-^t Bu-PhO^- =anion of 2,4-di-tertbutylphenol) ligands combined with the weak equatorial field of the ligand L^N6 , leads to [Dy^III (L^N6 )(2,4-di-^t Bu-PhO)₂ ](PF₆ ) (1), [Dy^III (L^N6 )(Ph₃ SiO)₂ ](PF₆ ) (2) and [Dy^III (L^N6 )(Ph₃ SiO)₂ ](BPh₄ ) (3) hexagonal bipyramidal dysprosium(III) single-molecule magnets (SMMs) with high anisotropy barriers of U_eff =973 K for 1, U_eff =1080 K for 2 and U_eff =1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher-order symmetry SMMs.

Keywords: ab initio calculations; dysprosium; hexagonal bipyramid; magnetic properties; single molecule magnets.

Figure 1

Molecular structure of 3 with Ph₃SiO⁻ as axial ligands.23 Upper Inset: The highlighted hexagonal bipyramidal core. Dy gold, O red, N blue, Si light turquoise, C gray, B dark yellow. Hydrogen atoms are omitted for clarity.

Scheme 1

Preparation of the precursor [Dy^IIIL^N6(CH₃CO₂)₂](CH₃CO₂)⋅9 H₂O (A) and 1–3.

Figure 2

Upper: Plots of χ _Μ′′(v) in zero applied dc field in the temperature range of 6–74 K for 3. Lower: Temperature dependence of the relaxation time for 1 (blue squares), 2 (green squares), and 3 (yellow squares), where the solid lines are fits of the data using the parameters given in the text.

Figure 3

Powder magnetic hysteresis measurements for 3 with an average sweep rate of 4 mT s⁻¹.

Figure 4

The direction of the principal anisotropy axis of the ground Kramers doublet for 3. Dy gold, O red, N blue, Si light turquoise, C gray, B dark yellow. Hydrogen atoms are omitted for clarity.

Figure 5

Ab initio calculated relaxation dynamics for complex 3. The arrows show the connected energy states with the number representing the matrix element of the transverse moment (see text for details). The black line indicates the KDs as function of magnetic moments. The red dashed arrow represents QTM (QTM=quantum tunneling of the magnetization) via ground state and TA‐QTM (TA‐QTM=thermally assisted QTM) via excited states. The violet dotted arrow indicates possible Orbach process. The pink thick arrow indicates the mechanism of magnetic relaxation. The numbers above each arrow represent corresponding transverse matrix elements for the transition magnetic moments.