A fluorobenzene-bound dysprosium half-sandwich dication single-molecule magnet

Dysprosium single-molecule magnets (SMMs) with two mutually <i>trans</i>-anionic ligands have shown large crystal field (CF) splitting, giving record effective energy barriers to magnetic reversal (<i>U</i> _eff) and hysteresis temperatures (<i>T</i> _H). However, these complexes tend to be bent, imposing a transverse field that reduces the purity of the <i>m</i> _<i>J</i> 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 <i>U</i> _eff and <i>T</i> _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 <i>U</i> _eff = 545(30) cm^-1 and <i>T</i> _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 <i>pseudo</i>-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*.