Dysprosium single-molecule magnets (SMMs) with two mutually trans-anionic ligands have shown large crystal field (CF) splitting, giving record effective energy barriers to magnetic reversal (U eff) and hysteresis temperatures (T H). However, these complexes tend to be bent, imposing a transverse field that reduces the purity of the m J projections of the CF states and promotes magnetic relaxation. A complex with only one charge-dense anionic ligand could have more pure CF states, and thus high U eff and T H. Here we report an SMM with this topology, a half-sandwich Dy(iii) complex [Dy(Cp*)(FPh)6][{Al[OC(CF3)3]3}2(μ-F)]2 (1-Dy; Cp* = C5Me5), and its Y(iii) analogue 1-Y; 1-Dy exhibits U eff = 545(30) cm-1 and T H = 14 K at sweep rates of 22 Oe s-1. The Cp* ligand imposes a strong axial CF, which is assisted by one axial fluorobenzene; the five equatorially-bound neutral fluorobenzenes present only weak transverse interactions to give a pseudo-pentagonal bipyramidal geometry. The salt metathesis reaction of 1-Y with KCp''' (Cp''' = {C5H2(SiMe3)3-1,2,4}) gave the sandwich complex [Y(Cp''')(Cp*)(FPh)2][{Al[OC(CF3)3]3}2(μ-F)] (4-Y), showing that the fluorobenzenes of 1-Y are easily displaced. We envisage that these methodologies could be adapted in future to prepare high-performance axial Dy SMMs with ligands that are more sterically demanding than Cp*.
This journal is © The Royal Society of Chemistry.