Probing relaxation dynamics in five-coordinate dysprosium single-molecule magnets

We report a new family of five-coordinate Lanthanide Single-Molecule Magnets (Ln SMMs) [Dy(Mes*O)2(THF)2X] (Mes*= 2,4,6-tri-tert-butylphenyl; X = Cl, 1; Br, 2; I, 3) with energy barriers to magnetic reversal amongst the highest reported for any SMM to date. The five-coordinate Dy(III) ions have distorted square pyramidal geometries, with halide anions on the apex, and two Mes*O ligands mutually trans- to each other, and the two THF molecules forming the second trans- pair. These geometrical features lead to a large magnetic anisotropy in these complexes giving Orbach energy barriers above 1200 K. QTM and Raman relaxation times are significantly enhanced by varying the apex halide from Cl to Br to I, or by simple dilution in a diamagnetic yttrium analogue.