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

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 (Ueff) and hysteresis temperatures (TH). However, these complexes tend to be bent, imposing a transverse field that reduces the purity of the mJ 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 Ueff and TH. Here we report an SMM with this topology, a half-sandwich Dy(III) complex [Dy(Cp*)(FPh)6][Al[OC(CF3)3]32(μ-F)]2 (1-Dy; Cp* = C5Me5), and its Y(III) analogue 1-Y; 1-Dy exhibits Ueff = 545(30) cm−1 and TH = 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]32(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*.