Excess axial electrostatic repulsion as a criterion for pentagonal bipyramidal DyIII single-ion magnets with high Ueff and TB

Abstract

In this work, a large excess of electrostatic repulsion, arising from the axial ligands, over that from the equatorial ligands is taken as the design strategy for high performance pentagonal bipyramidal (PBP) Dy^III single-ion magnets (SIMs). In this strategy, two PBP Dy^III-SIMs (1 and 2) [Dy(bbpen-CH₃)X] (X = Cl, 1; Br, 2; H₂bbpen = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)ethylenediamine) were synthesized and structurally characterized on the basis of a highly symmetrical ligand H₂bbpen-CH₃ in which an electron-donating group (–CH₃) was installed to favor the conformation necessary for the axial oxygen atom coordinating to Dy^III. Dynamic magnetic measurements verify the value of our design strategy since complex 2 exhibits high performance with large U_eff (above 1000 K) and a high magnetic hysteresis temperature (15 K). Ab initio calculations further verified the importance of the high excess of axial interaction which eventually leads to the special electronic structure possessing the desired magnetic properties. The search for excessive axial repulsion is not incompatible with the strategy based on local symmetry around the central ion since various high-performance PBP Dy^III-SIMs of both clearly distorted and nearly ideal geometries successfully acquire such a kind of excess. Apparently this study presents an alternative to the designing strategy for promising SIMs.